mfd: ab8500-debug: Function to save all ABB registers to mem

Dump function that stores all readable ABB registers to a memory
areas where they can be accessed from dump file.

Signed-off-by: Jonas Aaberg <jonas.aberg@stericsson.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Reviewed-by: Mattias WALLIN <mattias.wallin@stericsson.com>
Acked-by: Samuel Ortiz <sameo@linux.intel.com>