MAINTAINERS: add files for Mediatek DRM drivers

Mediatek HDMI 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>
Reviewed-by: Matthias Brugger <matthias.bgg@gmail.com>
Acked-by: Chunfeng Yun <chunfeng.yun@mediatek.com>