ARM: meson: switch AXG & GX dwc2 otg to DM

With CONFIG_DM_USB_GADGET=y, we no longer need any board_usb_*() logic
because:
* the dwc2 driver is DM compatible, and handles its own clock enabling
* the dwc3-meson-gxl glue drivers handles "force mode switching"

Remove all mach-meson gx & axg usb code and enable CONFIG_DM_USB_GADGET.

Note: Only configs having both CONFIG_USB_DWC3_MESON_GXL=y *and*
      USB_GADGET_DWC2_OTG=y have been updated.

Thanks for the original work from Mattijs at [1]

[1] https://lore.kernel.org/all/20221024-meson-dm-usb-v1-2-2ab077a503b9@baylibre.com/



Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Link: https://lore.kernel.org/r/20230117-u-boot-usb-gxl-otg-dm-v1-2-2853f6d75b06@linaro.org


Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>