dt-bindings: display: Convert VC4 bindings to schemas

The BCM283x SoCs have a display pipeline composed of several controllers
with device tree bindings that are supported by Linux.

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

This is just a 1:1 conversion though, and some bindings were incomplete so
it results in example validation warnings that are going to be addressed in
the following patches.

Acked-by: Eric Anholt <eric@anholt.net>
Reviewed: Rob Herring <robh+dt@kernel.org>
Signed-off-by: Maxime Ripard <maxime@cerno.tech>
Link: https://patchwork.freedesktop.org/patch/msgid/2dc6384c945c7d35ab4f75464d3a77046dc125b3.1590594512.git-series.maxime@cerno.tech