usb: otg: Add LPM support for USB PHY regulators

In some cases, when USB enters into Low Powr Mode, USB PHY
regulators can be turned off to save power.

If USB PHY is used for VBUS detection, USB PHY regulators can't be
turned off, but it can be put into LPM, to be able to exit from Low
Power Mode upon VBUS detection. This change add this capability.

Change-Id: I4981fa7b611c7b0aadc8deb66e6b7e7a42f9a44e
CRs-Fixed: 369499
Signed-off-by: Amit Blay <ablay@codeaurora.org>