regulator: Add WM831x EPE support

The WM831x series of PMICs provide two optional outputs for
controlling external devices during power sequencing, for example
an external regulator. While in essence these are GPIOs the
hardware presents them as DCDCs with very little control so
provide support via the regulator API in that fashion.

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>