dt-bindings: display: rockchip,inno-hdmi: Document GRF for RK3036 HDMI

HDMI on RK3036 use GRF control the HSYNC/VSYNC polarity, but this part
is missing when it first landing upstream.

Document that it is mandatory for RK3036 HDMI.

Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://lore.kernel.org/r/20250422070455.432666-3-andyshrk@163.com

Documentation/devicetree/bindings/display/rockchip/rockchip,inno-hdmi.yaml

@@ -56,6 +56,12 @@ properties:
       - port@0
       - port@1
 
+  rockchip,grf:
+    $ref: /schemas/types.yaml#/definitions/phandle
+    description:
+      Phandle to GRF used for control the polarity of hsync/vsync of rk3036
+      HDMI.
+
 required:
   - compatible
   - reg
@@ -75,6 +81,8 @@ allOf:
             const: rockchip,rk3036-inno-hdmi
 
     then:
+      required:
+        - rockchip,grf
       properties:
         power-domains: false
 
@@ -103,6 +111,7 @@ examples:
       clock-names = "pclk", "ref";
       pinctrl-names = "default";
       pinctrl-0 = <&hdmi_ctl>;
+      rockchip,grf = <&grf>;
       #sound-dai-cells = <0>;
 
       ports {