MT8173 HDMI support

- device tree binding documentation for MT8173 HDMI encoder, CEC, DDC,
  and PHY
- drivers for MT8173 HDMI encoder, CEC (HPD only for now), DDC, and PHY
- enable HDMI output via a custom SMCCC call
- add ddc-i2c-bus property to HDMI connector device tree binding

* tag 'mediatek-drm-2016-06-20' of git://git.pengutronix.de/git/pza/linux:
  dt-bindings: hdmi-connector: add DDC I2C bus phandle documentation
  drm/mediatek: enable hdmi output control bit
  drm/mediatek: Add HDMI support
  dt-bindings: drm/mediatek: Add Mediatek HDMI dts binding