mfd: add U300 AB3100 core support

This adds a core driver for the AB3100 mixed-signal circuit
found in the ST-Ericsson U300 series platforms. This driver
is a singleton proxy for all accesses to the AB3100
sub-drivers which will be merged on top of this one, RTC,
regulators, battery and system power control, vibrator,
LEDs, and an ALSA codec.

Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Reviewed-by: Mike Rapoport <mike@compulab.co.il>
Reviewed-by: Ben Dooks <ben-linux@fluff.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>