dt-bindings: mailbox: Add mediatek,gce-props.yaml

Add mediatek,gce-props.yaml for common GCE properties that is used for
both mailbox providers and consumers. We place the common property
"mediatek,gce-events" in this binding currently.

The property "mediatek,gce-events" is used for GCE event ID corresponding
to a hardware event signal sent by the hardware or a software driver.
If the mailbox providers or consumers want to manipulate the value of
the event ID, they need to know the specific event ID.

Signed-off-by: Jason-JH.Lin <jason-jh.lin@mediatek.com>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>