usb: dwc3: add Amlogic GXL & GXL DWC3 Glue

usb: dwc3: add Amlogic GXL & GXL DWC3 Glue

The USB support was initialy done with a set of PHYs and dwc3-of-simple
because the architecture of the USB complex was not understood correctly
at the time (and proper documentation was missing...).

But with the G12A family, the USB complex was correctly understood and
implemented correctly.

This adds a proper driver for the glue, based on the G12A one, but with
enough changes to require a different driver in U-Boot.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>