nvme: Add constants and types for the discovery log page

This is used in NVMe over Fabrics to enumerate a list of available
controllers.

Reviewed by: imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D44446

(cherry picked from commit 88ecf154c7c5a2e413a81ae1b0511b0295265b99)