regulator: spm-regulator: add support for a maximum voltage step size

Add support for a device tree property which defines the maximum
amount that the supply voltage is allowed to change in a single
step.  This feature can be utilized on devices which trigger
hardware recalibration sequences upon each voltage change.

Change-Id: I7088d4bb5b2edb7cd7934926babb5db76efba5d3
Signed-off-by: David Collins <collinsd@codeaurora.org>