i2c: cbus-gpio: Fix device tree binding

Looks like we're missing two lines needed to make it
work properly with device tree.

Signed-off-by: Tony Lindgren <tony@atomide.com>
Tested-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>