net/mlx5: Add multipath mode

In order to offload ecmp-on-host scheme where next-hop routes are used,
we will make use of HW LAG. Add accessor function to let upper layers
in the driver to realize if the lag acts in multi-path mode.

Signed-off-by: Roi Dayan <roid@mellanox.com>
Reviewed-by: Or Gerlitz <ogerlitz@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>