regulator: spm-regulator: Add support for HFS430 type regulators

Add support for PMIC HFSMPS 430 type regulators.  These have a
8 mV step size and a voltage control scheme consisting of two
8-bit registers defining a 16-bit voltage set point in units of
millivolts.

Change-Id: Ic4b38350f80384b288c77b60431affb666988a0d
Signed-off-by: Tirupathi Reddy <tirupath@codeaurora.org>