dt-bindings: usb: Convert Allwinner USB PHY controller to a schema

The Allwinner SoCs have a USB PHY controller that 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@cerno.tech>
Signed-off-by: Rob Herring <robh@kernel.org>