drm/mediatek: Add HDMI support

This patch adds drivers for the HDMI bridge connected to the DPI0
display subsystem function block, for the HDMI DDC block, and for
the HDMI PHY to support HDMI output.
This includes an interface to the generic hdmi-codec driver to start
or stop audio playback and to retrieve ELD (EDID like data) to limit the
supported audio formats to the HDMI sink capabilities.

Signed-off-by: Jie Qiu <jie.qiu@mediatek.com>
Signed-off-by: Junzhi Zhao <junzhi.zhao@mediatek.com>
Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>