mlxsw: Implement devlink interface

Implement newly introduced devlink interface. Add devlink port instances
for every port and set the port types accordingly.

Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>