mfd: Move arizona digital core supply management to the regulator API

Rather than open coding the enable GPIO control in the MFD core use the
API to push the management on to the regulator driver. The immediate
advantage is slight for most systems but this will in future allow device
configurations where an external regulator is used for DCVDD.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>