ARM: dts: sun8i: a83t: Add display pipeline

The display pipeline on the A83T is mainly composed of the mixers and
TCONs, plus various encoders.

Let's add the first mixer and TCON to the DTSI since the only board I have
can use only the LVDS output on the first TCON. The other parts will be
added eventually.

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