HW Relnotes: Add a section for Etherswitch

FreeBSD has quite a few drivers supporting several hardware Ethernet
switches. Add all(?) of them to the Hardware Release Notes template,
under a new category since this section is not the smallest section,
and Ethernet Interfaces is quite congested.

Discussed with: adrian (special thanks), bcr, mhorne
Reviewed by: carlavilla
Approved by: carlavilla (mentor)
Differential Revision: https://reviews.freebsd.org/D50303