usb: phy: msm-hsusb: Add workaround to keep LDOs on during LPM

There is a known HW issue on some targets that results in current
leakage through the PicoPHY if the VDD rail is ~1V and both the
1.8V and 3.1V rails are collapsed.

The recommended workaround is to leave the 1.8V/3.1V rails ON when
entering low power mode. Since the regulator enable() and disable()
calls are reference counted, this is easily done by making an
additional call to regulator_enable() to ensure the refcount is
nonzero. Add a device tree flag to enable this workaround.

Finally when the VDD voltage is minimized to its retention level
(~0.5 V) only then will the analog LDOs be allowed to be collapsed
to 0V as there will not be any leakage through the PHY. This case
specifically will be handled by the power management firmware in
order to achieve maximal power savings.

Change-Id: I6c2bee2ef93a15773365478ff9e09d92aa6b82dd
Signed-off-by: Jack Pham <jackp@codeaurora.org>