mfd: Use regcache_sync_region() to sync wm8994 GPIO registers on suspend

Now we have regcache sync region we can use it to do a more efficient
sync of the pin configuration after we reset the device during suspend.

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