irqchip: GICv3: ITS: plug ITS init into main GICv3 code

As the ITS is always a subsystem if GICv3, its probing/init is
driven by the main GICv3 code.

Plug that code in (guarded by a config option).

Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>
Link: https://lkml.kernel.org/r/1416839720-18400-12-git-send-email-marc.zyngier@arm.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>