dt-bindings: mailbox: Convert mtk-gce to DT schema

Convert the mtk-gce mailbox binding to DT schema format.

During the conversion, the examples for client device/mutex nodes
were removed, as these are found in their respective bindings:
arm/mediatek/mediatek,mmsys.yaml for "mediatek,mt8173-mmsys"
soc/mediatek/mediatek,mutex.yaml for "mediatek,mt8173-disp-mutex"

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220921090006.37642-1-angelogioacchino.delregno@collabora.com


Signed-off-by: Rob Herring <robh@kernel.org>