dt-bindings: display: mediatek,od: Add mediatek,gce-client-reg property

Currently, users of Mediatek OD (display overdrive) DT bindings set
mediatek,gce-client-reg node property, which is missing from the DT schema.

For example, device tree arch/arm64/boot/dts/mediatek/mt8173.dtsi is
causing the following dtb check error:

arch/arm64/boot/dts/mediatek/mt8173-elm.dtb: od@14023000 (mediatek,mt8173-disp-od): 'mediatek,gce-client-reg' does not match any of the regexes: '^pinctrl-[0-9]+$'

This commit adds the missing node property in the DT schema and updates the
example as well.

Signed-off-by: Ariel D'Alessandro <ariel.dalessandro@collabora.com>
Link: https://lore.kernel.org/r/20250911151001.108744-6-ariel.dalessandro@collabora.com
Signed-off-by: Rob Herring (Arm) <robh@kernel.org>

Documentation/devicetree/bindings/display/mediatek/mediatek,od.yaml

@@ -60,6 +60,18 @@ properties:
       - port@0
       - port@1
 
+  mediatek,gce-client-reg:
+    $ref: /schemas/types.yaml#/definitions/phandle-array
+    description: describes how to locate the GCE client register
+    items:
+      - items:
+          - description: Phandle reference to a Mediatek GCE Mailbox
+          - description:
+              GCE subsys id mapping to a client defined in header
+              include/dt-bindings/gce/<chip>-gce.h.
+          - description: offset for the GCE register offset
+          - description: size of the GCE register offset
+
 required:
   - compatible
   - reg
@@ -70,6 +82,7 @@ additionalProperties: false
 examples:
   - |
     #include <dt-bindings/clock/mt8173-clk.h>
+    #include <dt-bindings/gce/mt8173-gce.h>
 
     soc {
         #address-cells = <2>;
@@ -79,5 +92,6 @@ examples:
             compatible = "mediatek,mt8173-disp-od";
             reg = <0 0x14023000 0 0x1000>;
             clocks = <&mmsys CLK_MM_DISP_OD>;
+            mediatek,gce-client-reg = <&gce SUBSYS_1402XXXX 0x3000 0x1000>;
         };
     };