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

Documentation/devicetree/bindings/arm/rockchip.yaml

@@ -134,6 +134,7 @@ properties:
               - friendlyarm,nanopi-m4
               - friendlyarm,nanopi-m4b
               - friendlyarm,nanopi-neo4
+              - friendlyarm,nanopi-r4s
           - const: rockchip,rk3399
 
       - description: GeekBuying GeekBox