dt-bindings: arm: rockchip: Add Radxa ROCK Pi S

Add entry for the Radxa ROCK Pi S board
Signed-off-by: Sjoerd Simons <sjoerd@collabora.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220614064858.1445817-2-sjoerd@collabora.comSigned-off-by: Heiko Stuebner <heiko@sntech.de>

Documentation/devicetree/bindings/arm/rockchip.yaml

@@ -554,6 +554,11 @@ properties:
           - const: vamrs,rk3399pro-vmarc-som
           - const: rockchip,rk3399pro
 
+      - description: Radxa ROCK Pi S
+        items:
+          - const: radxa,rockpis
+          - const: rockchip,rk3308
+
       - description: Radxa Rock2 Square
         items:
           - const: radxa,rock2-square