drm/sun4i: Add HDMI support

The earlier Allwinner SoCs (A10, A10s, A20, A31) have an embedded HDMI
controller.

That HDMI controller is able to do audio and CEC, but those have been left
out for now.

Reviewed-by: Chen-Yu Tsai <wens@csie.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>