doc: driver-model: Convert fs_firmware_loader.txt to reST

Convert plain text documentation to reStructuredText format and add
it to Sphinx TOC tree. No essential content change.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>

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+.. SPDX-License-Identifier: GPL-2.0+
+.. Copyright (C) 2018-2019 Intel Corporation <www.intel.com>
+
+File System Firmware Loader
+===========================
+
+This is file system firmware loader for U-Boot framework, which has very close
+to some Linux Firmware API. For the details of Linux Firmware API, you can refer
+to https://01.org/linuxgraphics/gfx-docs/drm/driver-api/firmware/index.html.
+
+File system firmware loader can be used to load whatever(firmware, image,
+and binary) from the storage device in file system format into target location
+such as memory, then consumer driver such as FPGA driver can program FPGA image
+from the target location into FPGA.
+
+To enable firmware loader, CONFIG_FS_LOADER need to be set at
+<board_name>_defconfig such as "CONFIG_FS_LOADER=y".
+
+Firmware Loader API core features
+---------------------------------
+
+Firmware storage device described in device tree source
+-------------------------------------------------------
+For passing data like storage device phandle and partition where the
+firmware loading from to the firmware loader driver, those data could be
+defined in fs-loader node as shown in below:
+
+Example for block device::
+
+	fs_loader0: fs-loader {
+		u-boot,dm-pre-reloc;
+		compatible = "u-boot,fs-loader";
+		phandlepart = <&mmc 1>;
+	};
+
+<&mmc 1> means block storage device pointer and its partition.
+
+Above example is a description for block storage, but for UBI storage
+device, it can be described in FDT as shown in below:
+
+Example for ubi::
+
+	fs_loader1: fs-loader {
+		u-boot,dm-pre-reloc;
+		compatible = "u-boot,fs-loader";
+		mtdpart = "UBI",
+		ubivol = "ubi0";
+	};
+
+Then, firmware-loader property can be added with any device node, which
+driver would use the firmware loader for loading.
+
+The value of the firmware-loader property should be set with phandle
+of the fs-loader node. For example::
+
+	firmware-loader = <&fs_loader0>;
+
+If there are majority of devices using the same fs-loader node, then
+firmware-loader property can be added under /chosen node instead of
+adding to each of device node.
+
+For example::
+
+	/{
+		chosen {
+			firmware-loader = <&fs_loader0>;
+		};
+	};
+
+In each respective driver of devices using firmware loader, the firmware
+loaded instance	should be created by DT phandle.
+
+For example of getting DT phandle from /chosen and creating instance:
+
+.. code-block:: c
+
+	chosen_node = ofnode_path("/chosen");
+	if (!ofnode_valid(chosen_node)) {
+		debug("/chosen node was not found.\n");
+		return -ENOENT;
+	}
+
+	phandle_p = ofnode_get_property(chosen_node, "firmware-loader", &size);
+	if (!phandle_p) {
+		debug("firmware-loader property was not found.\n");
+		return -ENOENT;
+	}
+
+	phandle = fdt32_to_cpu(*phandle_p);
+	ret = uclass_get_device_by_phandle_id(UCLASS_FS_FIRMWARE_LOADER,
+					     phandle, &dev);
+	if (ret)
+		return ret;
+
+Firmware loader driver is also designed to support U-boot environment
+variables, so all these data from FDT can be overwritten
+through the U-boot environment variable during run time.
+
+For examples:
+
+storage_interface:
+  Storage interface, it can be "mmc", "usb", "sata" or "ubi".
+fw_dev_part:
+  Block device number and its partition, it can be "0:1".
+fw_ubi_mtdpart:
+  UBI device mtd partition, it can be "UBI".
+fw_ubi_volume:
+  UBI volume, it can be "ubi0".
+
+When above environment variables are set, environment values would be
+used instead of data from FDT.
+The benefit of this design allows user to change storage attribute data
+at run time through U-boot console and saving the setting as default
+environment values in the storage for the next power cycle, so no
+compilation is required for both driver and FDT.
+
+File system firmware Loader API
+-------------------------------
+
+.. code-block:: c
+
+	int request_firmware_into_buf(struct udevice *dev,
+				      const char *name,
+				      void *buf, size_t size, u32 offset)
+
+Load firmware into a previously allocated buffer
+
+Parameters:
+
+* struct udevice \*dev: An instance of a driver
+* const char \*name: name of firmware file
+* void \*buf: address of buffer to load firmware into
+* size_t size: size of buffer
+* u32 offset: offset of a file for start reading into buffer
+
+Returns:
+	size of total read
+	-ve when error
+
+Description:
+	The firmware is loaded directly into the buffer pointed to by buf
+
+Example of calling request_firmware_into_buf API after creating firmware loader
+instance:
+
+.. code-block:: c
+
+	ret = uclass_get_device_by_phandle_id(UCLASS_FS_FIRMWARE_LOADER,
+					     phandle, &dev);
+	if (ret)
+		return ret;
+
+	request_firmware_into_buf(dev, filename, buffer_location, buffer_size,
+				 offset_ofreading);