mfd: Add WM831x AUXADC support

The WM831x contains an auxiliary ADC with a number of switchable
inputs which is used to monitor some of the voltages and
temperatures in the system and has some external inputs which can be
used for machine specific purposes. Provide an API allowing drivers
to read values from the ADC.

An internal reference voltage is provided to allow callibration of
the ADC. This is used to calibrate the device at startup.

The hardware also supports continuous readings and digital comparators.
These are not yet supported by the driver.

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