irqchip/gic-v3: Add missing struct device_node declaration

When the GICv3 header file is used in a C file that doesn't include
any of the OF stuff, we end up with a bunch of ugly warnings.

Let's keep GCC quiet by adding a forward declaration.

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Cc: <linux-arm-kernel@lists.infradead.org>
Cc: Jason Cooper <jason@lakedaemon.net>
Link: http://lkml.kernel.org/r/1449483072-17694-2-git-send-email-marc.zyngier@arm.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>