hwmon: WM831x PMIC hardware monitoring driver

This driver adds support for the hardware monitoring features of
the WM831x PMICs to the hwmon API. Monitoring is provided for
the system voltages supported natively by the WM831x, the chip
temperature, the battery temperature and the auxiliary inputs
of the WM831x.

Currently no alarms are supported, though digital comparators on
the WM831x devices would allow these to be provided.

Since the auxiliary and battery temperature input scaling depends
on the system configuration the value is reported as a voltage to
userspace.

Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Acked-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>