mfd: Make use of the ab8500 firmware read-modify-write service

This patch updates the AB8500 driver to make use of the I2C
read-modify-write service in the PRCMU firmware.

Signed-off-by: Mattias Nilsson <mattias.i.nilsson@stericsson.com>
Reviewed-by: Mattias Wallin <mattias.wallin@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>