NgATM: deprecate

Support for ATM NICs was removed prior to FreeBSD 12. At the time it
was unclear if this code was still useful without it. Now the time has
come to finish removing support.

Add DREPRECATION NOTICEs to the manpages and gone_in(14, ...) warnings
in the constructors (or module init for ngatmbase).

MFC after: 3 days
Relnotes: yes

Reviewed by: manu, emaste
Differential Revision: https://reviews.freebsd.org/D38877

(cherry picked from commit 211c302f10d76fc12e50c87039cfc06aa9e659c1)