mfd: Use regmap for tps65910 register access.

Using regmap apis for accessing the device registers and
using RBTREE caching mechanims for caching registers.

Enabling caching of the registers which is used for voltage
controls. By doing this, the modify_bits operation is faster as
it does not involve the i2c register read from device, just read
from cache. This results faster set voltage operation.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Reviewed-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>
diff --git a/drivers/mfd/Kconfig b/drivers/mfd/Kconfig
index d92bea2..16178e1 100644
--- a/drivers/mfd/Kconfig
+++ b/drivers/mfd/Kconfig
@@ -177,6 +177,7 @@
 	depends on I2C=y && GPIOLIB
 	select MFD_CORE
 	select GPIO_TPS65910
+	select REGMAP_I2C
 	help
 	  if you say yes here you get support for the TPS65910 series of
 	  Power Management chips.