dt-bindings: arm: mediatek: mmsys: add power and gce properties

Power:
1. Add description for power-domains property.

GCE:
1. Add description for mboxes property.
2. Add description for mediatek,gce-client-reg property.

Signed-off-by: jason-jh.lin <jason-jh.lin@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Link: https://lore.kernel.org/r/20220419033237.23405-2-rex-bc.chen@mediatek.com
Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>