msm: board-msm7x27a-regulator: Add voltage selector for a regulator

Each regulator in PMIC support a range of voltage over which
it can be operated. This voltage range is changed in certain
minimum voltage step and these steps provide the voltage selector
points for a particular regulator. This range is used by
regulator_count_voltages() function.

Change-Id: I958170f35779eb3d84faad6502c3e2f14b1cafb6
Signed-off-by: Pankaj Kumar <pakuma@codeaurora.org>