cxl: docs/platform/acpi/srat Add generic target documentation

Add description in the SRAT document to describe the Generic Port
Affinity sub-table.

Reviewed-by: Gregory Price <gourry@gourry.net>
Reviewed-by: Alison Schofield <alison.schofield@intel.com>
Link: https://patch.msgid.link/20250515000923.2590820-3-dave.jiang@intel.com


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