mfd: ab8500-debugfs: add debugfs node to read all registers

Update the ab8500_registers_print() to reuse it from multiple places.

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>