net/mlx5: separate Netlink command interface

net/mlx5: separate Netlink command interface

The Netlink commands interfaces is included in the mlx5.h file with a
lot of other PMD interfaces.

As an arrangement to make the Netlink commands shared with different
PMDs, this patch moves the Netlink interface to a new file called
mlx5_nl.h.

Move non Netlink pure vlan commands from mlx5_nl.c to the
mlx5_vlan.c.

Rename Netlink commands and structures to use prefix mlx5_nl.

Signed-off-by: Matan Azrad <matan@mellanox.com>
Acked-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>