regulator: spm-regulator: Add support for FTS426 type regulators

Add support for PMIC FTSMPS 426 type regulators.  These have a
4 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.

Also address the mode register configuration differences between
FTSMPS 426 type regulators and other regulators supported in
spm-regulator driver file.

CRs-Fixed: 2037603
Change-Id: Ide54c8c9dd8113ab4588979ec97915403b3ecab1
Signed-off-by: Tirupathi Reddy <tirupath@codeaurora.org>