usb: phy: qusb: Add support for notifier for power down

On platform where there is a support of regulator
shutdown in case of an event, and if the regulator is
shared with the PHY, it will result in an unintended
disconnect.
So, tear the complete USB stack in that scenario and
bring the stack up only if that event has been subsided.

Change-Id: Ibf60079620ed8c1f319b7e1b5f63cab1f5b2f399
Signed-off-by: Sriharsha Allenki <sallenki@codeaurora.org>