irqchip: gic: Support hierarchy irq domain.

Add support to use gic as a parent for stacked irq domain.

Signed-off-by: Yingjoe Chen <yingjoe.chen@mediatek.com>
Acked-by: Marc Zyngier <marc.zyngier@arm.com>
Link: https://lkml.kernel.org/r/1416902662-19281-2-git-send-email-yingjoe.chen@mediatek.com


Signed-off-by: Jason Cooper <jason@lakedaemon.net>