mfd: palmas: Add APIs to access the Palmas' registers

Palmas register set is divided into different blocks (base and offset)
and hence different i2c addresses. The i2c address offsets are derived
from base address of block of registers.

Add inline APIs to access the Palma's registers which takes the base of
register block and register offset. The i2c address offset is  derived
from the base address of register blocks.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>