leds: ktd2692: Make aux-gpios optional

Make the AUX pin optional, since it isn't a core part of functionality,
and the device is designed to be operational with only one CTRL pin.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Signed-off-by: Markuss Broks <markuss.broks@gmail.com>
Signed-off-by: Pavel Machek <pavel@ucw.cz>