MFC r333499

Add deprecation notice for vxge.

This driver was merged to HEAD one week prior to Exar publicly
announcing theyhad left the Ethernet market. It is not known to be used
and has various code quality issues spotted by Brooks and Hiren. Retire
it in preparation for FreeBSD 12.0.

Submitted by: kbowling
Reported by: rgrimes
Approved by: re (gjb)
Relnotes: yes