ACPI: property: Allow direct graph endpoint references

By using device and further data node references, allow direct references
to endpoints. These are of form

	Package() { \DEV, "portX", "endpointY" }

where X is the number of the port and Y is the number of the endpoint.

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