net/mlx5: Rename vport QoS group reference to parent

Rename the `group` field in the `mlx5_vport` structure to `parent` to
clarify the vport's role as a member of a parent group and distinguish
it from the concept of a general group.

Additionally, rename `group_entry` to `parent_entry` to reflect this
update.

This distinction will be important for handling more complex group
structures and scheduling elements.

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>