dt-bindings: mailbox: add GCE header file for mt8188

Add Global Command Engine(GCE) header file to define the GCE thread priority,
GCE subsys id, event and constant for mt8188.

Signed-off-by: Elvis Wang <Elvis.Wang@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno<angelogioacchino.delregno@collabora.com>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>