net/mlx5: DR, Add CONFIG_MLX5_SW_STEERING for software steering support

Add new mlx5 Kconfig flag to allow selecting software steering
support and compile all the steering files only if the flag is
selected.

Signed-off-by: Alex Vesker <valex@mellanox.com>
Signed-off-by: Yevgeny Kliteynik <kliteyn@mellanox.com>
Reviewed-by: Erez Shitrit <erezsh@mellanox.com>
Reviewed-by: Mark Bloch <markb@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>