mfd: Enable register cache for wm8994 devices

As part of this we provide information about the registers that exist in
the device to the regmap core, drop the small amount of cache that the
core had been using and let regmap do the sync.

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