dt-bindings: Add doc for FriendlyARM NanoPi R2C

Add devicetree binding documentation for the FriendlyARM NanoPi R2C.

Signed-off-by: Tianling Shen <cnsztl@gmail.com>
Reviewed-by: Jagan Teki <jagan@amarulasolutions.com>
Acked-by: Krzysztof Kozlowski <krzk@kernel.org>
Link: https://lore.kernel.org/r/20230325074022.9818-2-cnsztl@gmail.com


Signed-off-by: Heiko Stuebner <heiko@sntech.de>