cxl: docs/platform/cdat reference documentation

Add documentation for CDAT structures for CXL usages.

Reviewed-by: Gregory Price <gourry@gourry.net>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Link: https://patch.msgid.link/20250515000923.2590820-2-dave.jiang@intel.com


Signed-off-by: Dave Jiang <dave.jiang@intel.com>