mfd: ab8500: Fix dt irq mapping

The AD8500 defines itself as interrupt-controller in DT,
but it doesn't assign DT node to IRQ domain when creates it.
As result, of_irq_xx() helpers don't work because they can't
find necessary IRQ domain.

Hence, fix it by assigning AD8500 core device DT node to IRQ
domain when it's created.

This patch fixes STE u8500 Snowball boot failure reported by Kevin Hilman
https://lkml.org/lkml/2014/5/27/624

Reported-and-tested-by: Kevin Hilman <khilman@linaro.org>
Signed-off-by: Grygorii Strashko <grygorii.strashko@ti.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>