irqchip/mips-gic: Add device hierarchy domain

Now the root gic_irq_domain is split into device and IPI domains.

This form provides a better representation of how the root domain is split into
2. One for devices and one for IPIs.

Signed-off-by: Qais Yousef <qais.yousef@imgtec.com>
Acked-by: Ralf Baechle <ralf@linux-mips.org>
Cc: <jason@lakedaemon.net>
Cc: <marc.zyngier@arm.com>
Cc: <jiang.liu@linux.intel.com>
Cc: <linux-mips@linux-mips.org>
Cc: <lisa.parratt@imgtec.com>
Cc: Qais Yousef <qsyousef@gmail.com>
Link: http://lkml.kernel.org/r/1449580830-23652-14-git-send-email-qais.yousef@imgtec.com


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>