regulator: Add WM831x ISINK support

The WM831x series of PMICs provide two constant current sinks
designed to drive strings of serially connected LEDs for applications
such as backlights. This driver adds support for those regulators.

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>