dt-bindings: display: vop2: Add missing rockchip,grf property for rk3566/8

The clock polarity of RGB signal output is controlled by GRF, this
property is already being used in the current device tree, but
forgot to describe it as a required property in the binding file.

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20250303034436.192400-6-andyshrk@163.com

Documentation/devicetree/bindings/display/rockchip/rockchip-vop2.yaml

@@ -146,6 +146,9 @@ allOf:
         rockchip,vop-grf: false
         rockchip,pmu: false
 
+      required:
+        - rockchip,grf
+
   - if:
       properties:
         compatible:
@@ -200,6 +203,7 @@ examples:
                               "dclk_vp1",
                               "dclk_vp2";
                 power-domains = <&power RK3568_PD_VO>;
+                rockchip,grf = <&grf>;
                 iommus = <&vop_mmu>;
                 vop_out: ports {
                     #address-cells = <1>;