msm: krait-regulator-pmic: Add krait-regulator-pmic driver

Read the register configurations of regulators in the gang and
bug if the gang is not setup correctly.

Importantly, fix the voltage overshoots observed during PFM-PWM
transitions by implementing the following steps.

PFM -> PWM
After the buck is set to PWM mode, wait 50uS and then set the PWM
current limit to 3.5A.

PWM -> PFM
After the buck is set to pfm mode, immediately lower the PWM current limit
to 2A (no delay required).

Change-Id: I15af09aaf8427e75253a4e12878ee1f0a1265454
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>