ARM: dts: sun6i: a31: Enable HDMI support on the Mele I7

The Mele I7 has an HDMI connector wired to the HDMI pins
on the SoC. Enable the display pipeline and HDMI output.

Signed-off-by: Marcus Cooper <codekipper@gmail.com>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
[wens@csie.org: Added "dts" prefix]
Signed-off-by: Chen-Yu Tsai <wens@csie.org>