mfd: Enable rbtree cache for wm831x devices

Most useful with the regulators where we're doing a lot of read/modify/write
updates in potentially performance critical paths.  Providing some defaults
would make this slightly better but this is a win right now.

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