mfd: Add IRQ domain support for the AB8500

As the AB8500 is an IRQ controller in its own right, here we provide
the AB8500 driver with IRQ domain support. This is required if we wish
to reference any of its IRQs from a platform's Device Tree.

Cc: Naga Radheshy <naga.radheshy@stericsson.com>
Cc: Mattias Wallin <mattias.wallin@stericsson.com>
Cc: Daniel Willerud <daniel.willerud@stericsson.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Samuel Ortiz <sameo@linux.intel.com>