ARM: meson: g12a: switch 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-g12a glue drivers handles "force mode switching"

Remove all mach-meson/g12a usb code and enable CONFIG_DM_USB_GADGET.

Note: Only configs having both CONFIG_USB_DWC3_MESON_G12A=y *and*
      USB_GADGET_DWC2_OTG=y have been updated.

Signed-off-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Link: https://lore.kernel.org/r/20221024-meson-dm-usb-v1-2-2ab077a503b9@baylibre.com


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