usb: phy-msm-qusb: Support low power mode

Enables support for low power mode for QUSB2 PHY.
Allows XO shutdown during bus suspend.

Change-Id: I5ee42112f7c63c957d00889acb26931d2b555f9e
Signed-off-by: Mayank Rana <mrana@codeaurora.org>
Signed-off-by: Azhar Shaikh <azhars@codeaurora.org>