usb: udc: implement DM versions of usb_gadget_initialize()/_release()/_handle_interrupt()

When DM_USB_GADGET the platform code for the USB device must be replaced by
calls to a USB device driver.

usb_gadget_initialize() probes the USB device driver.
usb_gadget_release() removes the USB device driver.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@ti.com>
Reviewed-by: Lukasz Majewski <lukma@denx.de>