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