mfd: Support software initiated shutdown of WM831x PMICs

In systems where there is no hardware signal from the processor to the
PMIC to initiate the final power off sequence we must initiate the
shutdown with a register write to the PMIC. Support such systems in the
driver. Since this may prevent a full shutdown of the system platform
data is used to enable the feature.

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