dt-bindings: Add doc for FriendlyARM NanoPi R2S

Add devicetree binding documentation for the FriendlyARM NanoPi R2S.

Signed-off-by: David Bauer <mail@david-bauer.net>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200920154528.88185-1-mail@david-bauer.net


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