ARM: dts: sun6i: Add device node for HDMI controller

Now that we support the HDMI controller on the A31 SoC, we can add it
to the device tree.

This adds a device node for the HDMI controller, and the of_graph nodes
connecting it to the 2 TCONs.

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