BCM2708: armctrl: Add IRQ Device Tree support

Add Device Tree IRQ support for BCM2708.
Usage is the same as for irq-bcm2835.
See binding document: brcm,bcm2835-armctrl-ic.txt

A bank 3 is added to handle GPIO interrupts. This is done because
armctrl also handles GPIO interrupts.

Signed-off-by: Noralf Tronnes <notro@tronnes.org>