Add deprecation notice for lmc(4)

We intend to remove support before FreeBSD 12 is branched. These are
available only as 32-bit PCI devices. The driver has an ambiguous
license and I have not been successful in contacting the driver's author
in order to address this.

The planned deprecation has been announced on -current and -stable; if
we receive feedback that the driver is still useful and we are able to
resolve the license issue this deprecation notice can be reverted.

Reviewed by: bapt, brooks, imp, rgrimes
MFC after: 2 weeks
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D15182