dt-bindings: bus: Convert Allwinner DE2 bus to a schema

The Allwinner SoCs using the second generation of the display engine have a
bus for that display engine. The bus is supported in Linux, with a matching
Device Tree binding.

Now that we have the DT validation in place, let's convert the device tree
bindings for that controller over to a YAML schemas.

Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
[robh: add 'type: object' for child nodes]
Signed-off-by: Rob Herring <robh@kernel.org>