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
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