iwlwifi: add sysctl to dump PCI IDs/ names / firmware-prefixes

Add a FreeBSD-specifc SYSCTL_PROC to dump the list of
PCI IDs / name / firmware-prefix which can be post-processed
for man pages (iwlwifi.4 and iwlwififw.4) or the wiki.

The output still yields duplicates depending on by what information
you need so one may wants to filter them out (try not to use sort/uniq).

Sponsored by: The FreeBSD Foundation
MFC after: 3 days