net/mlx5: Add parent group support in rate group structure

Introduce a `parent` field in the `mlx5_esw_rate_group` structure to
support hierarchical group relationships.

The `parent` can reference another group or be set to `NULL`,
indicating the group is connected to the root TSAR.

This change enables the ability to manage groups in a hierarchical
structure for future enhancements.

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>