net/mlx5: add VXLAN to flow translate routine

net/mlx5: add VXLAN to flow translate routine

This part of patchset adds support of VXLAN-related items and
actions to the flow translation routine. Later some tunnel types,
other than VXLAN can be addedd (GRE). No VTEP devices are created at
this point, the flow rule is just translated, not applied yet.

Suggested-by: Adrien Mazarguil <adrien.mazarguil@6wind.com>
Signed-off-by: Viacheslav Ovsiienko <viacheslavo@mellanox.com>
Acked-by: Yongseok Koh <yskoh@mellanox.com>