arm_ffa: introduce Arm FF-A support

Add Arm FF-A support implementing Arm Firmware Framework for Armv8-A v1.0

The Firmware Framework for Arm A-profile processors (FF-A v1.0) [1]
describes interfaces (ABIs) that standardize communication
between the Secure World and Normal World leveraging TrustZone
technology.

This driver uses 64-bit registers as per SMCCCv1.2 spec and comes
on top of the SMCCC layer. The driver provides the FF-A ABIs needed for
querying the FF-A framework from the secure world.

The driver uses SMC32 calling convention which means using the first
32-bit data of the Xn registers.

All supported ABIs come with their 32-bit version except FFA_RXTX_MAP
which has 64-bit version supported.

Both 32-bit and 64-bit direct messaging are supported which allows both
32-bit and 64-bit clients to use the FF-A bus.

FF-A is a discoverable bus and similar to architecture features.
FF-A bus is discovered using ARM_SMCCC_FEATURES mechanism performed
by the PSCI driver.

Clients are able to probe then use the FF-A bus by calling the DM class
searching APIs (e.g: uclass_first_device).

The Secure World is considered as one entity to communicate with
using the FF-A bus. FF-A communication is handled by one device and
one instance (the bus). This FF-A driver takes care of all the
interactions between Normal world and Secure World.

The driver exports its operations to be used by upper layers.

Exported operations:

- ffa_partition_info_get
- ffa_sync_send_receive
- ffa_rxtx_unmap

Generic FF-A methods are implemented in the Uclass (arm-ffa-uclass.c).
Arm specific methods are implemented in the Arm driver (arm-ffa.c).

For more details please refer to the driver documentation [2].

[1]: https://developer.arm.com/documentation/den0077/latest/
[2]: doc/arch/arm64.ffa.rst

Signed-off-by: Abdellatif El Khlifi <abdellatif.elkhlifi@arm.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Ilias Apalodimas <ilias.apalodimas@linaro.org>
Cc: Tom Rini <trini@konsulko.com>
Cc: Jens Wiklander <jens.wiklander@linaro.org>
Cc: Heinrich Schuchardt <xypron.glpk@gmx.de>

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