dwc3_msm: Support Low Power Mode with HSPHY on USB disconnect

Put HSPHY into RETENTION on cable disconnect. This allows USB
driver to release wakelock and turn-off clocks, regulators etc.
Also add support for hs_phy_irq which is used by DWC3 to notify
wakeup events e.g. BSVHV interrupts on cable connect via separate
interrupt line.

Change-Id: I91bc2c05ffdb7fb145b9bce5f1431f38e166d3fe
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>