nvd.4: Add a HARDWARE section for HW Relnotes

Approved by: re (cperciva)
MFC after: 3 days
Reviewed by: dab
Differential Revision: https://reviews.freebsd.org/D52867

(cherry picked from commit 7f5267a99895a797ea135b67aa58d561ee52b279)
(cherry picked from commit 795f4eef2c56e8f6609f008d4af35f45232602e8)