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>

drivers/mfd/wm831x-core.c

@@ -1602,6 +1602,8 @@ struct regmap_config wm831x_regmap_config = {
 	.reg_bits = 16,
 	.val_bits = 16,
 
+	.cache_type = REGCACHE_RBTREE,
+
 	.max_register = WM831X_DBE_CHECK_DATA,
 	.readable_reg = wm831x_reg_readable,
 	.writeable_reg = wm831x_reg_writeable,