mfd: Function for obtaining the db8500 prcmu firmware version

This patch exports a function that can be used to tell which
version of the DB8500 PRCMU firmware is available, and revamps the
firmware detection code a bit.

Reviewed-by: Bengt Jonsson <bengt.g.jonsson@stericsson.com>
Reviewed-by: Jonas Aberg <jonas.aberg@stericsson.com>
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>