ACPI: bus: Add iterator for dependent devices

Add a helper macro to iterate over ACPI devices that are flagged
as consumers of an initial supplier ACPI device.

Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Daniel Scally <djrscally@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>