mitigations.7: explain installing firmware + spdx

MFC after: 3 days
Reported by: imp (ucode is for security)
Reported by: emaste (ucode is not minix)
Reported by: delphij (please ucode asap)
Reviewed by: imp
Pull Request: https://github.com/freebsd/freebsd-src/pull/1411

(cherry picked from commit b15aff050530a791262e166ee0c8fed3a118e7d6)