pkgbase: Create a mlx-tools package

mlx* are only useful for users who have a Mellanox card.
Create a package for it so users that don't have this card can
avoid having this program.

Differential Revision: https://reviews.freebsd.org/D31795

(cherry picked from commit 80645e1ce5574ce4d63f0a1cad287949440384da)