irqchip: Move ARM GIC to drivers/irqchip

From: Rob Herring <rob.herring at calxeda.com>

Now that we have drivers/irqchip, move GIC irqchip to drivers/irqchip. This
is necessary to share the GIC with arm and arm64.

Signed-off-by: Rob Herring <rob.herring at calxeda.com>
Cc: Russell King <linux at arm.linux.org.uk>
Cc: Thomas Gleixner <tglx at linutronix.de>