mfd: ab5500 chip register access

The analog baseband chip ab5500 is a multi functional chip
containing regulators, charging, gpio, USB and accessory detect.
It also contain various multimedia functionalities like digital
encoder and audio codec.
The core driver added with this patch provides register access via
i2c via PRCMU. Event handling implemented as irq_chip will come in
future patches since it depends on PRCMU functionality not yet
implemented.

Signed-off-by: Mattias Wallin <mattias.wallin@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>