ACPI / property: Expose data-only subnodes via sysfs

Add infrastructure needed to expose data-only subnodes of ACPI
device objects introduced previously via sysfs.

Each data-only subnode is represented as a sysfs directory under
the directory corresponding to its parent object (a device or a
data-only subnode).  Each of them has a "path" attribute (containing
the full ACPI namespace path to the object the subnode data come from)
at this time.

Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Tested-by: Mika Westerberg <mika.westerberg@linux.intel.com>

include/acpi/acpi_bus.h

@@ -382,9 +382,12 @@ struct acpi_device {
 /* Non-device subnode */
 struct acpi_data_node {
 	const char *name;
+	acpi_handle handle;
 	struct fwnode_handle fwnode;
 	struct acpi_device_data data;
 	struct list_head sibling;
+	struct kobject kobj;
+	struct completion kobj_done;
 };
 
 static inline bool acpi_check_dma(struct acpi_device *adev, bool *coherent)