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

Documentation/devicetree/bindings/arm/rockchip.yaml

@@ -201,6 +201,11 @@ properties:
               - friendlyarm,nanopi-r4s-enterprise
           - const: rockchip,rk3399
 
+      - description: FriendlyElec NanoPi R5S board
+        items:
+          - const: friendlyarm,nanopi-r5s
+          - const: rockchip,rk3568
+
       - description: GeekBuying GeekBox
         items:
           - const: geekbuying,geekbox