usb: dwc-meson-g12a: Add support for USB on AXG SoCs

The Amlogic AXG is close to the GXL Glue but with a single OTG PHY.

It needs the same init sequence as GXL & GXM, but it seems it doesn't need
the host disconnect bit.

Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Kevin Hilman <khilman@baylibre.com>
Signed-off-by: Felipe Balbi <balbi@kernel.org>