usb: dwc3: of-simple: remove Amlogic GXL and AXG compatibles

There is now a dedicated driver for these SoCs making the old compatible
obsolete.

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