HID: wiimote: Add Nintendo Balance-Board support

The Nintendo Balance-Board is a controller which behaves exactly like the
Wii Remote but reports all its data through a special extension device.
Hence, we can simply add the Balance-Board as extension device and we get
full support for it.

Tested-by: Florian Echtler <floe@butterbrot.org>
Signed-off-by: David Herrmann <dh.herrmann@googlemail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>