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

Link: https://patchwork.kernel.org/project/linux-mediatek/patch/20220504091923.2219-3-rex-bc.chen@mediatek.com/Signed-off-by: Xinlei Lee <xinlei.lee@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Rex-BC Chen <rex-bc.chen@mediatek.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>