In an RCH topology a CXL host-bridge as Root Complex Integrated Endpoint
the represents the memory expander. Unlike a VH topology there is no
CXL/PCIE Root Port that host the endpoint. The CXL subsystem maps this
as the CXL root object (ACPI0017 on ACPI based systems) targeting the
host-bridge as a dport, per usual, but then that dport directly hosts
the endpoint port.

Mock up that configuration with a 4th host-bridge that has a 'cxl_rcd'
device instance as its immediate child.
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Link: https://lore.kernel.org/r/166993046170.1882361.12460762475782283638.stgit@dwillia2-xfh.jf.intel.comReviewed-by: Robert Richter <rrichter@amd.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>