ACPI / DSD: Document references, ports and endpoints

Document the use of references into the hierarchical data extension
structure, as well as the use of port and endpoint concepts that are very
similar to those in Devicetree.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>