genirq: Add a new function to get IPI reverse mapping

When dealing with coprocessors we need to find out the actual hwirqs values to
pass on to the firmware so that it knows what it needs to use to receive IPIs
from and send IPIs to Linux cpus.

[ tglx: Fixed the single hwirq IPI case. The hardware irq number does not
  	change due to the cpu number ]

Signed-off-by: Qais Yousef <qais.yousef@imgtec.com>
Cc: <jason@lakedaemon.net>
Cc: <marc.zyngier@arm.com>
Cc: <jiang.liu@linux.intel.com>
Cc: <ralf@linux-mips.org>
Cc: <linux-mips@linux-mips.org>
Cc: <lisa.parratt@imgtec.com>
Cc: Qais Yousef <qsyousef@gmail.com>
Link: http://lkml.kernel.org/r/1449580830-23652-10-git-send-email-qais.yousef@imgtec.com


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