dt-bindings: Add doc for FriendlyARM NanoPi R4S

Add devicetree binding documentation for the FriendlyARM NanoPi R4S.

Signed-off-by: Tianling Shen <cnsztl@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20210319051627.814-1-cnsztl@gmail.com


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