iwlwifi: add man pages

Add and hook up man pages for iwlwifi and iwlwififw and install a copy
of the firmware license to /usr/share/docs/legal so it will always be
shipped with the installed system.

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 5a02ae5e8f2d74b7ac20fa2e0cd98c66d934905b)