iwm.4: add iwlwifi cross-reference

iwlwifi(4) supports a superset of the devices supported by iwm(4). The
latter may be retired in the future (if there is no reason to prefer it
for the set of devices supported by both).

Sponsored by: The FreeBSD Foundation

(cherry picked from commit 9dd5023e955dac6bf1d4372d1b611a85f8b6f508)