mfd: Add regmap cache support for wm8350

Use the most simple possible transformation on the existing code so keep
the table sitting around, further patches in this series will delete the
existing cache code - the main purpose of this patch is to ensure that
we always have a cache for bisection.

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