ACPI: property: Make the ACPI graph API private

The fwnode graph API is preferred over the ACPI graph API. Therefore
make the ACPI graph API private, and use it as a back-end for the
fwnode graph API only.

Unused functionality is removed while the functionality actually used
remains the same.

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