documentation: dt: omap: crossbar: Add description for interrupt consumer

The current crossbar description does not include the description
required for the consumer of the crossbar, a.k.a devices whoes events
pass through the crossbar into the GIC interrupt controller.

So, provide documentation for the same.

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/1403766634-18543-16-git-send-email-r.sricharan@ti.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>