mfd: ab8500-core: Allow the possibility to dump all AB8500 registers

Implement an API so that a user may dump all AB8500 registers
via debugfs file access.

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>