net/mlx5: Add ability to insert to specific flow group

If the flow table isn't an autogroup the upper driver has to create the
flow groups explicitly. This information can't later be used when
creating rules to insert into a specific flow group. Allow such use case.

Signed-off-by: Mark Bloch <mbloch@nvidia.com>
Reviewed-by: Maor Gottlieb <maorg@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>