net/mlx5: Add devlink interface

The devlink interface is initially used to set/get the mode of the SRIOV e-switch.

Currently, these are only stubs for get/set, down-stream patch will actually
fill them out.

Signed-off-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>