tools/testing/cxl: Add a physical_node link

Emulate what ACPI does to link a host bridge platform firmware device to
device node on the PCI bus. In this case it's just self referencing
link, but it otherwise lets the tooling test out its lookup code.

Link: https://lore.kernel.org/r/164298433209.3018233.18101085948127163720.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <dan.j.williams@intel.com>