mfd: AB8500 register access via PRCMU I2C

This patch adds the choice of accessing the AB8500 registers
via prcmu I2C. Access either via SPI or I2C is supported.

Signed-off-by: Mattias Wallin <mattias.wallin@stericsson.com>
Acked-by: Linus Walleij <linus.walleij@stericsson.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>