ACPI: property: graph: Update graph documentation to use generic references

Instead of port and endpoint properties for representing ports and
endpoints, use the keys of the hierarchical data extension references
when referring to the port and endpoint nodes. Additionally, use "reg"
properties as in Device Tree to specify the number of the port or the
endpoint.

The keys of the port nodes begin with "port" and the keys of the endpoint
nodes begin with "endpoint", both followed by "@" character and the number
of the port or the endpoint.

These changes have the advantage that no ACPI specific properties need to
be added to refer to non-device nodes. Additionally, using the name of the
node instead of an integer property inside the node is easier to parse in
code and easier for humans to understand.

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