regulator: qpnp-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.

Change-Id: Id27bf066a014c0a39f47febff2603873050125d9
CRs-Fixed: 986619
Signed-off-by: David Collins <collinsd@codeaurora.org>