drm/mediatek: add hdmi driver for MT2701 and MT7623

This patch adds hdmi dirver suppot for both MT2701 and MT7623.
And also support other (existing or future) chips that use
the same binding and driver.

Signed-off-by: chunhui dai <chunhui.dai@mediatek.com>
Signed-off-by: CK Hu <ck.hu@mediatek.com>