irqchip: crossbar: Dont use '0' to mark reserved interrupts

Today '0' is actually reserved, but may not be the same in the future.

So, use a flag to mark the GIC interrupts that are reserved.

Signed-off-by: Nishanth Menon <nm@ti.com>
Signed-off-by: Sricharan R <r.sricharan@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Link: https://lkml.kernel.org/r/1403766634-18543-2-git-send-email-r.sricharan@ti.com


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