dt-bindings: add bindings for DE2 on V3s SoC

Allwinner V3s SoC have a display engine which have a different pipeline
with older SoCs.

Add document for it (new compatibles and the new "mixer" part).

Signed-off-by: Icenowy Zheng <icenowy@aosc.io>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>