dt-bindings: display: sun4i: Add binding for A31 HDMI controller

The HDMI controller in the A31 SoC is slightly different from the
earlier version. In addition to the TMDS clock and DDC controls,
this version now takes a second DDC clock input.

Add a compatible string for it, and add the DDC clock input to the
list of clocks required.

Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171010032008.682-7-wens@csie.org