mfd: Refactor DB8500 PRCMU reg access

Instead of carrying around the __PRCMU_BASE in every read or
write to the PRCMU registers, move it out to the register
definition file and define registers along with their base
offset so that the code gets easier to read.

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