drm/meson: Add support for HDMI encoder and DW-HDMI bridge + PHY

The Amlogic Meson GXBB/GXL/GXM SoCs embeds a Synopsys DesignWare HDMI TX
Controller with a custom Bridge + PHY around the Controller.

This driver makes uses of all the custom PHY plat data callbacks and enables
the compatible HDMI modes to be configured as a drm_encoder instance.

Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>