ACPI: property: Document key numbering for hierarchical data extension refs

As part of the hierarchical data extension key naming, introduce numbering
scheme for the nodes that may be referred to using hierarchical data
extension references. This allows iterating over particular kind of nodes
recognised by the node name whilst allowing numbering the nodes, bringing
ACPI to feature parity with DT in this respect.

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