mfd: Initial core support for WM831x series devices

The WM831x series of devices are register compatible processor power
management subsystems, providing regulator and power path management
facilities along with other services like watchdog, RTC and touch
panel controllers.

This patch adds very basic support, providing basic single register
I2C access, handling of the security key and registration of the
devices.

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