usb: dwc2: gadget: LPM interrupt handler

This interrupt indicates that an LPM transaction
was received on the USB bus. After getting this
interrupt we are going from L0 state to L1 state.

Signed-off-by: Sevak Arakelyan <sevaka@synopsys.com>
Signed-off-by: Grigor Tovmasyan <tovmasya@synopsys.com>
Signed-off-by: Felipe Balbi <felipe.balbi@linux.intel.com>