nvme: Add SGL structure and constants for use in NVMe commands

Fabrics capsules use an SGL structure instead of prp1/2 addresses to
describe the data buffer used for a command. The SGL structure is
added to a union with the existing prp1/2 fields.

Reviewed by: imp
Sponsored by: Chelsio Communications
Differential Revision: https://reviews.freebsd.org/D44442