regulator: Add WM831x DC-DC buck convertor support

The WM831x series of devices all have 3 DC-DC buck convertors. This
driver implements software control for these regulators via the
regulator API.  Use with split hardware/software control of individual
regulators is not supported, though regulators not controlled by
software may be controlled via the hardware control interfaces.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Liam Girdwood <lrg@slimlogic.co.uk>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>