MAINTAINERS: add files for Mediatek DRM drivers

Mediatek MIPI DSI phy driver is moved from drivers/gpu/drm/mediatek to
drivers/phy/mediatek, so add the new folder to the Mediatek DRM drivers'
information.

Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>
Acked-by: Chunfeng Yun <chunfeng.yun@mediatek.com>