genirq: Add ipi_offset to irq_common_data

IPIs are always assumed to be consecutively allocated, hence virqs and hwirqs
can be inferred by using CPU id as an offset. But the first cpu doesn't always
have to start at offset 0. ipi_offset stores the position of the first cpu so
that we can easily calculate the virq or hwirq of an IPI associated with a
specific cpu.

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/1449580830-23652-6-git-send-email-qais.yousef@imgtec.com


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