regulator: Versatile Express regulator driver

Implementation of the regulator framework driver for the
Versatile Express voltage control. Devices without
voltage constraints (ie. "regulator-[min|max]-microvolt"
properties in the DT node) are treated as fixed (or rather
read-only) regulators.

Signed-off-by: Pawel Moll <pawel.moll@arm.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>