It's possible that the controller has an individually switchable power supply.
Therefore add support to control a supplying regulator.

As this is not always the case, the regulator is requested as optional.
Signed-off-by: Heiko Stuebner <heiko.stuebner@bq.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>