ng_atmllc: deprecate
With the deprecation of NgATM, ng_atmllc is the last vestage of ATM
support so deprecate it too.
MFC after: 3 days
Relnotes: yes
Reviewed by: manu, emaste
Differential Revision: https://reviews.freebsd.org/D38878
(cherry picked from commit c2285a7b092f524b5694e42bfb5fd24f71815771)