mfd: Convert AB3100 driver to threaded IRQ

This converts the AB3100 core MFD driver to use a threaded
interrupt handler instead of the explicit top/bottom-half
construction with a workqueue. This saves some code and make it
more similar to other modern MFD drivers.

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