cxl: Introduce parent_port_of() helper

cxl: Introduce parent_port_of() helper

Often a parent port must be determined. Introduce the parent_port_of()
helper function to avoid open coding of determination of a parent
port.

Signed-off-by: Robert Richter <rrichter@amd.com>
Reviewed-by: Gregory Price <gourry@gourry.net>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Reviewed-by: "Fabio M. De Francesco" <fabio.m.de.francesco@linux.intel.com>
Tested-by: Gregory Price <gourry@gourry.net>
Acked-by: Dan Williams <dan.j.williams@intel.com>
Link: https://patch.msgid.link/20250509150700.2817697-5-rrichter@amd.com
Signed-off-by: Dave Jiang <dave.jiang@intel.com>