mfd: Add a prcmu_abb_write_masked routine to db8500-prcmu

This patch adds driver support for the I2C read-modify-write
service in the U8500 PRCMU firmware.

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