genirq: Irqchip: document gcflags arg of irq_alloc_domain_generic_chips

Commit 088f40b7b027dad6519712ff224a5798dd62a204 ("genirq: Generic chip:
Add linear irq domain support") missed kerneldoc for the gcflags
argument of irq_alloc_domain_generic_chips(). Add it now.

Signed-off-by: James Hogan <james.hogan@imgtec.com>
Acked-by: Grant Likely <grant.likely@secretlab.ca>
Link: http://lkml.kernel.org/r/1371564513-4327-1-git-send-email-james.hogan@imgtec.com
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>