mfd: Convert WM831x SPI to use dev_pm_ops

There is a move to deprecate bus-specific PM operations and move to
using dev_pm_ops instead in order to reduce the amount of boilerplate
code in buses and facilitiate updates to the PM core. Do this move for
the WM831x SPI driver.

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