mfd: Add wm8994 register access and default information

Describe the register map to the regmap core so that we can use its
diagnostic features and cache support.  This is split out from the patch
using it due to the size so that the actual code change is a bit clearer.

As the various devices are supersets of each other the access maps are
built up by layering the functions on top of each other, though the
interface for specifying the register defaults isn't currently amenable
to this.

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