MT8173 DRM support

- device tree binding documentation for all MT8173 display
  subsystem components
- basic mediatek-drm driver for MT8173 with two optional,
  currently fixed output paths:
- DSI encoder support for DSI and (via bridge) eDP panels
- DPI encoder support for output to HDMI bridge
- necessary clock tree changes for the DPI->HDMI path
- export mtk-smi functions used by mediatek-drm
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