drm/mediatek: add the mipitx driving control

Add a property in device tree to control the driving by different
board.

Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Signed-off-by: Jitao Shi <jitao.shi@mediatek.com>
Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>