dt-bindings: display: Add compatible for A64 DE2 display pipeline

Allwinner A64 has a DE2 display pipeline. The TCONs are similar to the
ones in A83T, but the mixers are new (similar to the later R40 SoC).

This patch adds dt-binding documentation for A64 DE2 display pipeline.

Signed-off-by: Jagan Teki <jagan@amarulasolutions.com>
Reviewed-by: Rob Herring <robh@kernel.org>
[Icenowy: Refactor and also cover TCON1]
Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180904044053.15425-4-icenowy@aosc.io