net/mlx5: Introduce node type to rate group structure

Introduce the `sched_node_type` enum to represent both the group and
its members as scheduling nodes in the rate hierarchy.

Add the `type` field to the rate group structure to specify the type of
the node membership in the rate hierarchy.

Generalize comments to reflect this flexibility within the rate group
structure.

Signed-off-by: Carolina Jubran <cjubran@nvidia.com>
Reviewed-by: Cosmin Ratiu <cratiu@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Reviewed-by: Daniel Machon <daniel.machon@microchip.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>