Add the mlx5 and mlx5en modules to the i386 and amd64 kernel builds by
default and add a manual page for mlx5en. The mlx5 module contains
shared code for both infiniband and ethernet. The mlx5en module
contains specific code for ethernet functionality only. A mlx5ib
module is in the works for infiniband support.

Supported hardware:

• ConnectX-4: 10/20/25/40/50/56/100Gb/s speeds.
• ConnectX-4 LX: 10/25/40/50Gb/s speeds (low power consumption)

Refer to the mlx5en(4) manual page for a comprehensive list.

The team porting the mlx5 driver(s) to FreeBSD:

• Hans Petter Selasky <hselasky@freebsd.org>
• Oded Shanoon <odeds@mellanox.com>
• Meny Yossefi <menyy@mellanox.com>
• Shany Michaely <shanim@mellanox.com>
• Shahar Klein <shahark@mellanox.com>
• Daria Genzel <dariaz@mellanox.com>
• Mark Bloch <markb@mellanox.com>

Differential Revision: https://reviews.freebsd.org/D4163
Submitted by: Mark Block <markb@mellanox.com>
Sponsored by: Mellanox Technologies
Reviewed by: gnn @
MFC after: 3 days