regulator: Add WM831x DC-DC boost convertor support

The WM831x series of PMICs include a single DC-DC boost convertor.
This adds basic support for this convertor.

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>