usb: msm_otg: Add support for separate ASYNC IRQ to exit LPM

Some USB cores have a separate IRQ line for issuing ASYNC
interrupts in LPM. This interrupt needs to be enabled only
when hardware is put in low power state.

Change-Id: Ie7de237ccbaa90294b20d3fce594ec280cb1641a
Signed-off-by: Manu Gautam <mgautam@codeaurora.org>