mfd: ab8500-core switch off status added

This patch adds a sysfs file with the ab8500 switch off status.
The switch off status contains information of what caused the ab8500
chip to power off. A print during boot is also added.

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