ARM: multi_v7_defconfig: enable Rockchip io-domain driver

The io-domain driver is necessary to notify the soc about voltages changes
happening on supplying regulators. Probably the most important user right
now is the mmc tuning code, where the soc needs to get notified when the
voltage is dropped to the 1.8V point.

As this option is necessary to sucessfully tune UHS cards etc, it should
get built in.

Signed-off-by: Heiko Stuebner <heiko.stuebner@collabora.com>
Signed-off-by: Olof Johansson <olof@lixom.net>