irqchip: crossbar: Add kerneldoc for crossbar_domain_unmap callback

Adding kerneldoc for unmap callback function.

Signed-off-by: Sricharan R <r.sricharan@ti.com>
Acked-by: Santosh Shilimkar <santosh.shilimkar@ti.com>
Link: https://lkml.kernel.org/r/1403766634-18543-13-git-send-email-r.sricharan@ti.com


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