mfd: Create a common interface for dbx500 PRCMU drivers

This adds a header file that contains the set of functions and
definitions that will be shared between the DB8500 and DB5500
PRCMU drivers.
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>