irqchip: GICv3: ITS: irqchip implementation

The usual methods that are used to present an irqchip to the rest
of the kernel

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Link: https://lkml.kernel.org/r/1416839720-18400-6-git-send-email-marc.zyngier@arm.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>