ARM: dts: sun6i: Enable HDMI support on some A31/A31s devices

All the A31/A31s devices I own have some kind of HDMI connector wired
to the dedicated HDMI pins on the SoC:

  - A31 Hummingbird (standard HDMI connector, display already enabled)
  - Sinlinx SinA31s (standard HDMI connector)
  - MSI Primo81 tablet (micro HDMI connector)

Enable the display pipeline (if needed) and HDMI output for them.

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