nl80211: advertise GTK rekey support, new triggers

Since we now have the necessary API in place to support
GTK rekeying, applications will need to know whether it
is supported by a device. Add a pseudo-trigger that is
used only to advertise that capability. Also, add some
new triggers that match what iwlagn devices can do.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>