Add dt-binding documentation of dpi for MediaTek MT8186 SoC.

Link: https://patchwork.kernel.org/project/linux-mediatek/patch/1649645584-13186-2-git-send-email-xinlei.lee@mediatek.com/Signed-off-by: Xinlei Lee <xinlei.lee@mediatek.com>
Reviewed-by: Rex-BC Chen <rex-bc.chen@mediatek.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>