irqchip: GICv3: ITS: enable compilation of the ITS driver

Get the show on the road...

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Link: https://lkml.kernel.org/r/1416839720-18400-13-git-send-email-marc.zyngier@arm.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>