This introduces a new sub-device module for the BATTERY handlers. It
moves the whole power_supply battery handling over to wiimote-modules.

This doesn't change any semantics or ABI but only converts the battery
handling into a sub-device module.
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>