ACPI: property: Document hierarchical data extension references

Add documentation on how to refer to hierarchical data nodes in a
generic way. This brings ACPI to feature parity with Device Tree in
terms of being able to refer to any node in the tree.
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

Documentation/acpi/dsd/data-node-references.txt

+Copyright (C) 2018 Intel Corporation
+Author: Sakari Ailus <sakari.ailus@linux.intel.com>
+
+
+Referencing hierarchical data nodes
+-----------------------------------
+
+ACPI in general allows referring to device objects in the tree only.
+Hierarchical data extension nodes may not be referred to directly, hence this
+document defines a scheme to implement such references.
+
+A reference consist of the device object name followed by one or more
+hierarchical data extension [1] keys. Specifically, the hierarchical data
+extension node which is referred to by the key shall lie directly under the
+parent object i.e. either the device object or another hierarchical data
+extension node.
+
+Example
+-------
+
+	In the ASL snippet below, the "reference" _DSD property [2] contains a
+	device object reference to DEV0 and under that device object, a
+	hierarchical data extension key "node" referring to the NODE object and
+	lastly, a hierarchical data extension key "anothernode" referring to the
+	ANOD object which is also the final target of the reference.
+
+	Device (DEV0)
+	{
+	    Name (_DSD, Package () {
+		ToUUID("dbb8e3e6-5886-4ba6-8795-1319f52a966b"),
+		Package () {
+		    Package () { "node", NODE },
+		}
+	    })
+	    Name (NODE, Package() {
+		ToUUID("dbb8e3e6-5886-4ba6-8795-1319f52a966b"),
+		Package () {
+		    Package () { "anothernode", ANOD },
+		}
+	    })
+	    Name (ANOD, Package() {
+		ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
+		Package () {
+		    Package () { "random-property", 0 },
+		}
+	    })
+	}
+
+	Device (DEV1)
+	{
+	    Name (_DSD, Package () {
+		ToUUID("daffd814-6eba-4d8c-8a91-bc9bbf4aa301"),
+		Package () {
+		    Package () { "reference", ^DEV0, "node", "anothernode" },
+		}
+	    })
+	}
+
+
+References
+----------
+
+[1] Hierarchical Data Extension UUID For _DSD.
+    <URL:http://www.uefi.org/sites/default/files/resources/_DSD-hierarchical-data-extension-UUID-v1.1.pdf>,
+    referenced 2018-07-17.
+
+[2] Device Properties UUID For _DSD.
+    <URL:http://www.uefi.org/sites/default/files/resources/_DSD-device-properties-UUID.pdf>,
+    referenced 2016-10-04.