Modify is_acpi_node() to return "true" for ACPI data-only subnodes as
well as for ACPI device objects and change the name of to_acpi_node()
to to_acpi_device_node() so it is clear that it covers ACPI device
objects only.  Accordingly, introduce to_acpi_data_node() to cover
data-only subnodes in an analogous way.

With that, make the fwnode_property_* family of functions work with
ACPI data-only subnodes introduced previously.
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Mika Westerberg <mika.westerberg@linux.intel.com>