hwmon: Add WM835x PMIC hardware monitoring driver

This driver provides reporting of the status supply voltage rails
of the WM835x series of PMICs via the hwmon API.

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>