wlan_gcmp.4: add man page

The man page is heavily based on wlan_ccmp.4 with minor modifications
for GCMP.

Add the .Xr to the other wlan*.4 man pages.

Sposnored by: The FreeBSD Foundation
MFC after: 3 days
Reviewed by: adrian, ziaee
Differential Revision: https://reviews.freebsd.org/D50789