dt-bindings: usb: dwc2: Add Amlogic G12A DWC2 Compatible

Adds the specific compatible string for the DWC2 IP found in the
Amlogic G12A SoC Family.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>