dt-bindings: display: sun4i: Add HDMI display bindings

One of the possible output of the display pipeline, on the SoCs that have
it, is the HDMI controller.

Add a binding for it.

Acked-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>