ARM: ux500: Force AB8500 to use the GIC as its interrupt controller

It's understood that the AB8500 should be subordinate to the DB8500;
however, the AB8500 uses the GIC as it's interrupt controller. If
we do not specify which IRQ controller to use the default is to use
the next encountered IRQ controller as we climb the tree. This would
be the DB8500. This patch ensures the AB8500 makes use of the correct
interrupt controller.

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>