mfd: Put WM8994 into reset when suspending

Ensure that the chip is in the lowest power mode possible when suspended
by performing a soft reset on it. On early silicon revisions the lowest
power modes can't be entered without using reset so we can't achieve
equivalent results within the individual drivers.

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