usb: sandbox: Add DM_USB_GADGET support

Remove local usb_gadget_register_driver()/usb_gadget_unregister_driver()
implementation which is implemented in udc-core.c instead if DM_USB_GADGET
is enabled. Add no-op dm_usb_gadget_handle_interrupts() implementation.

Reviewed-by: Mattijs Korpershoek <mkorpershoek@baylibre.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Signed-off-by: Marek Vasut <marex@denx.de>