ARM: ixp4xx: Switch to use new IRQ+GPIO drivers

This deletes the old irq+gpiochip combo from the IXP4xx
machine and switches it over to use the new drivers merged
in respective subsystem.

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