mfd: db8500-prcmu reset code retrieval

This implements the reset code retrieval function so we can
ipso facto get to know how the system was reset.

Signed-off-by: Sebastian Rasmussen <sebastian.rasmussen@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>