mfd: Remove custom wm8350 cache implementation

Since none of the users now reference the cache directly we can happily
remove the custom cache code and rely on the regmap cache.

For simplicity we don't bother with the register defaults tables but
instead read the defaults from the device - regmap is capable of doing
this, unlike our old cache infrastructure. This saves a lot of code and
allows us to cache the device revision information too.

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