dt-bindings: Add doc for FriendlyARM NanoPi R5S

Add devicetree binding documentation for the FriendlyARM NanoPi R5S.

Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20230308063240.107178-1-anarsoul@gmail.com


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