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 <[email protected]>
Reviewed-by: AngeloGioacchino Del Regno<[email protected]>
Acked-by: Krzysztof Kozlowski <[email protected]>
Signed-off-by: Jassi Brar <[email protected]>

Author: Elvis Wang