dt-bindings: usb: dwc3: Move Amlogic G12A DWC3 Glue Bindings to YAML schemas

Now that we have the DT validation in place, let's convert the device tree
bindings for the Amlogic G12A DWC3 Glue Bindings over to a YAML schemas,
the AXG and GXL glue bindings will be converted later.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
[robh: drop maxItems on vbus-supply]
Signed-off-by: Rob Herring <robh@kernel.org>