drm/meson: Add G12A support for the DW-HDMI Glue

The Amlogic G12A embeds the same Synopsys DW-HDMI Controller,
but with :
- a "backport" of the HDR signaling registers from more recent
  DW-HDMI controllers, this will need a tweak since it's not
  normally present on this version of the DW-HDMI controller
- A direct mapping of TOP and DW-HDMI registers instead of an
  internal bus accessed using read/write registers
- Support for RX-SENSE, but not yet implemented
- Support for HDMI 2.1 Dynamic HDR, but not yet implemented
- Different registers mapping for the HDMI PHY setup

This patchs adds support for these changes while providing exact
same support as the previous GXBB, GXL & GXM SoCs.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Tested-by: Jerome Brunet <jbrunet@baylibre.com>
Reviewed-by: Jerome Brunet <jbrunet@baylibre.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190325141824.21259-12-narmstrong@baylibre.com