RDMA/mlx5: Update SPDX tags to show proper license

Mellanox code is supposed to be OpenIB compliant code,
so let's update SPDX tags to show it.

Fixes: fc385b7a ("IB/mlx5: Add basic regiser/unregister representors code")
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>