cxl/core/port: Clarify decoder creation

Add wrappers for the creation of decoder objects at the root level and
switch level, and keep the core helper private to cxl/core/port.c. Root
decoders are static descriptors conveyed from platform firmware (e.g.
ACPI CFMWS). Switch decoders are CXL standard decoders enumerated via
the HDM decoder capability structure. The base address for the HDM
decoder capability structure may be conveyed either by PCIe or platform
firmware (ACPI CEDT.CHBS).

Additionally, the kdoc descriptions for these helpers and their
dependencies is updated.

Signed-off-by: Ben Widawsky <ben.widawsky@intel.com>
[djbw: fixup changelog, clarify kdoc]
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/164366463014.111117.9714595404002687111.stgit@dwillia2-desk3.amr.corp.intel.com


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