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

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>