usb: dwc3: meson-g12a: add support for GXL and GXM SoCs

In order to add support for the Amlogic GXL/GXM USB Glue, this adds
the corresponding :
- PHY names
- clock names
- USB2 PHY init and mode set
- regmap setup

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