dt-bindings: display: amlogic, meson-dw-hdmi: Add G12A compatible and ports

The Amlogic G12A SoC has a slighly modified DW-HDMI Glue with
support for HDMI 2.1 and a different DW-HDMI register access.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20190313141030.5958-3-narmstrong@baylibre.com