msm: spm-regulator: add support for mode selection

Add support to the spm-regulator driver to select which regulator
operating mode to utilize.  There are three possible mode
selections: PWM mode, auto mode, or the existing mode set by
hardware or bootloaders.

Change-Id: Ieaf69f26078d4e9f14db3b2eb7b5cbe2dc8ed31e
Signed-off-by: David Collins <collinsd@codeaurora.org>