dt-bindings: display: mediatek: add vdosys1 MERGE property for mt8195

MT8195 vdosys1 merge1 to merge4 have HW mute function.
Add MERGE additional mute property description.

Signed-off-by: Nancy.Lin <nancy.lin@mediatek.com>
Acked-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>