board: rockchip: add Anbernic RGXX3 Series Devices

The Anbernic RGxx3 is a "pseudo-device" that encompasses the following
devices:

 - Anbernic RG353M
 - Anbernic RG353P
 - Anbernic RG353V
 - Anbernic RG353VS
 - Anbernic RG503

The rk3566-anbernic-rgxx3.dtsi is synced with upstream Linux, but
rk3566-anbernic-rgxx3.dts is a U-Boot specific devicetree that
is used for all RGxx3 devices.

Via the board.c file, the bootloader automatically sets the correct
fdtfile, board, and board_name environment variables so that the
correct devicetree can be passed to Linux. It is also possible to
simply hard-code a single devicetree in the boot.scr file and use
that to load Linux as well.

The common specifications for each device are:

 - Rockchip RK3566 SoC
 - 2 external SDMMC slots
 - 1 USB-C host port, 1 USB-C peripheral port
 - 1 mini-HDMI output
 - MIPI-DSI based display panel
 - ADC controlled joysticks with a GPIO mux
 - GPIO buttons
 - A PWM controlled vibrator
 - An ADC controlled button

All of the common features are defined in the devicetree synced from
upstream Linux.

TODO: DSI panel auto-detection for the RG353 devices (requires porting
of DSI controller driver and DSI-DPHY driver to send DSI commands to
the panel).

Signed-off-by: Chris Morgan <macromorgan@hotmail.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>

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