irqchip: ixp4xx: Add OF initialization support

This adds support for probing and settin up the IXP4xx
irqchip from device tree.

Cc: Jason Cooper <jason@lakedaemon.net>
Cc: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>