usb: dwc2: gadget: Add handler for WkupAlert interrupt

Added interrupt handler for WkupAlert interrupt.

This interrupt should initiate Remote Wake up.

Acked-by: Minas Harutyunyan <hminas@synopsys.com>
Signed-off-by: Grigor Tovmasyan <tovmasya@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>