cxl/core: Generalize dport enumeration in the core

The core houses infrastructure for decoder resources. A CXL port's
dports are more closely related to decoder infrastructure than topology
enumeration. Implement generic PCI based dport enumeration in the core,
i.e. arrange for existing root port enumeration from cxl_acpi to share
code with switch port enumeration which just amounts to a small
difference in a pci_walk_bus() invocation once the appropriate 'struct
pci_bus' has been retrieved.

Set the convention that decoder objects are registered after all dports
are enumerated. This enables userspace to know when the CXL core is
finished establishing 'dportX' links underneath the 'portX' object.

Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/164368114191.354031.5270501846455462665.stgit@dwillia2-desk3.amr.corp.intel.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>