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>