mfd: AB3550 core driver

This adds a core driver for the AB3550 mixed-signal circuit
found in the ST-Ericsson U300 platforms. This driver
is a singleton proxy for all access to the AB3550 sub
functionality drivers which can be added on top of this one:
RTC, regulators, battery and system power control, vibrator,
LEDs and an ALSA codec.

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