rtc: AB3100 RTC support

This adds support for the RTC found inside the AB3100 Mixed Signal chip.
The symbols used for communicating with the chip is found in the
mfd/ab3100-core.c driver that also provides the platform device.

Signed-off-by: Linus Walleij <linus.walleij@stericsson.com>
Acked-by: Alessandro Zummo <alessandro.zummo@towertech.it>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>