firmware: move firmware_class from Documentation/ to samples/

Move the firmware_class sample drivers to samples/ so that they are
buildable and can be maintained.

Signed-off-by: Randy Dunlap <randy.dunlap@oracle.com>
Acked-by: Marcel Holtmann <marcel@holtmann.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>