usb: dwc3: meson-g12a: handle the phy and glue registers separately

On the Amlogic GXL/GXM SoCs, only the USB control registers are available,
the PHY mode being handled in the PHY registers.

Thus, handle the PHY mode registers in separate regmaps and prepare
support for Amlogic GXL/GXM SoCs by moving the regmap setup in a callback
set in the SoC match data.

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