IB/mlx5: Expose MPLS related tunneling offloads

This patch reports the device's capbilities to offload
encapsulated MPLS tunnel protocols to user-space:
- Capability to offload MPLS over GRE.
- Capability to offload MPLS over UDP.

Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Ariel Levkovich <lariel@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>