usb: dwc2: gadget: enable WKUP_ALERT interrupt

WKUP_ALERT interrupt should be unmask when lpm mode is enabled.

This interrupt is asserted when the device is in L1 for the duration
mentioned in GREFCLK.SOF_CNN_WKUP_ALERT. This is used to alert SW to
initiate Remote wake up so that the device resumes in time in order not
to lose sync with the host frame number.

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>