dt-bindings: Add doc for FriendlyARM NanoPi R4S Enterprise Edition

Add devicetree binding documentation for the FriendlyARM NanoPi R4S
Enterprise Edition.

Signed-off-by: Tianling Shen <cnsztl@gmail.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220915022511.4267-1-cnsztl@gmail.com


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