dt-bindings: display: Convert Allwinner display pipeline to schemas

The Allwinner SoCs have a display engine composed of several controllers
assembled differently depending on the SoC, the number and type of output
they have, and the additional features they provide. A number of those are
supported in Linux, with the matching bindings.

Now that we have the DT validation in place, let's split into separate file
and convert the device tree bindings for those controllers to schemas.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/20200103152801.47254-1-maxime@cerno.tech