msm: krait-regulator: Switch to PFM mode under light load

The pmic gang can operate in low power mode when configured
in PFM mode.  In PFM mode the number of phases drop to one
where the gang leader provides all the current.

Use this facility when the combined load decreases below
500mA.

Change-Id: I7e975f811cf1f7cf38300d6c92fd3234d9d0b13f
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>