mfd: abx500-core: Provide an API to dump all ABx500 registers

Some drivers can detect subsystem failures e.g. shared memory driver
can detect modem sub system failures. It would be helpful in analyzing
these failures if AB register dump is available at that point. This
patch adds the API for the drivers to dump AB registers in the kernel
log.

Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Mian Yousaf Kaukab <mian.yousaf.kaukab@stericsson.com>
Reviewed-by: Linus WALLEIJ <linus.walleij@stericsson.com>
Reviewed-by: Jonas ABERG <jonas.aberg@stericsson.com>