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 <[e-mail]>
Reviewed-by: Matthias Brugger <[e-mail]>
Acked-by: Chunfeng Yun <[e-mail]>

MAINTAINERS

···
 S:	Supported
 F:	Documentation/devicetree/bindings/display/mediatek/
 F:	drivers/gpu/drm/mediatek/
+F:	drivers/phy/mediatek/phy-mtk-hdmi*
 
 DRM DRIVERS FOR NVIDIA TEGRA
 M:	Thierry Reding <[e-mail]>