usb: dwc3-meson-gxl: force mode on child add/removal

arch/mach-meson has some custom usb logic, in particular:
* on board_usb_init(), we force USB_DR_MODE_PERIPHERAL
* on board_usb_cleanup(), we force USB_DR_MODE_HOST

With DM_USB_GADGET=y, board_usb_init/cleanup() are no
longer used when we call usb_gadget_initialize().
Instead, the generic (from udc-uclass) initialization/release is
called, which itself calls the controller driver's probe()/remove().

Therefore, enabling DM_USB_GADGET=y will mean that this mode
switching will break.

To prepare for enabling DM_USB_GADGET, perform the mode switching
when the "amlogic,meson-g12a-usb" (dwc2) driver is
probed()/removed() instead.

This is achieved via the glue driver, which gets notified each time one
of its children is probed()/removed.

Note: this change should be harmless without DM_USB_GADGET=y
because the amlogic-g12a-usb driver is not probed via driver model.

Thanks for Mattijs for the original work at [1].

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

Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Link: https://lore.kernel.org/r/20230117-u-boot-usb-gxl-otg-dm-v1-1-2853f6d75b06@linaro.org
Signed-off-by: Neil Armstrong <neil.armstrong@linaro.org>

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