mfd: Align ab8500 with the abx500 interface

This patch makes the ab8500 mixed signal chip expose the same
interface for register access as the ab3100, ab3550 and ab5500 chip.
The ab8500_read() and ab8500_write() is removed and replaced with
abx500_get_register_interruptible() and
abx500_set_register_interruptible().

Signed-off-by: Mattias Wallin <mattias.wallin@stericsson.com>
Acked-by: Linus Walleij <linus.walleij@stericsson.com>
Acked-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>