mfd: Remove wm8400 custom cache implementation

Save a useful amount of code by removing the custom cache implementation
for wm8400 and using the regmap cache. Also simplify things by not
separately reseting the CODEC registers, this is a sufficiently infrequent
operation that we can simply invalidate the entire cache when this happens.

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