dt-bindings: soc: mediatek: add gce-client-reg for MUTEX

In order to allow modules with latency requirements such as MDP3
to set registers through CMDQ, add the relevant GCE property.

Signed-off-by: Moudy Ho <moudy.ho@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: CK Hu <ck.hu@mediatek.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220610063424.7800-5-moudy.ho@mediatek.com


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>