ng_atmllc: deprecate

With the deprecation of NgATM, ng_atmllc is the last vestage of ATM
support so deprecate it too.

Reviewed by: manu, emaste
Relnotes: yes
Differential Revision: https://reviews.freebsd.org/D38878

(cherry picked from commit c2285a7b092f524b5694e42bfb5fd24f71815771)