mfd: Refactor wm831x AUXADC handling into a separate file

In preparation for some additional work on the wm831x AUXADC code move the
support into a separate file. This is a simple code motion patch, there
should be no functional changes.

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