dt-bindings: Add doc for FriendlyARM NanoPi R5C

Add devicetree binding documentation for the FriendlyARM NanoPi R5C.

Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Tianling Shen <cnsztl@gmail.com>
Link: https://lore.kernel.org/r/20230318083745.6181-3-cnsztl@gmail.com


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