usb: dwc3: add support for pwr_event_irq

Adds support for the pwr_event_irq in general, and for
the L1_OUT pwr event specifically. The support consists of:
1. Enabling the pwr_event_irq based on the DT.
2. Adding a threaded irq handler as well as a centralized function
   (dwc3_pwr_event_handler()) for handling the various power events.
3. Enabling the L1_OUT pwr event. This is done on an
   arch-by-arch basis. The trigger for enabling this is
   dbm_l1_lpm_interrupt().
4. Calling usb_gadget_wakeup() in order to initiate a wakeup sequence
   without sending a remote-wakeup indication to the host.
5. Adding a call from dwc3_msm_resume() to dwc3_pwr_event_handler().
   This is done in order to overcome a situation where
   dwc3_msm_suspend() would have disabled the clocks, thereby
   disabling the ability to read the PWR_STAT register before calling
   the handler function.

Change-Id: Ia548ec2e9ee4a7c6634c87ba24fe4cdbf1221000
Signed-off-by: Dov Levenglick <dovl@codeaurora.org>
Signed-off-by: Bar Weiner <bweiner@codeaurora.org>