ARM: dts: imx7s: Specify #io-channel-cells in ADC nodes

Specify #io-channel-cells in ADC nodes. Needed to be able to reference
them by phandle.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Cc: Shawn Guo <shawnguo@kernel.org>
Cc: Chris Healy <cphealy@gmail.com>
Cc: Andrew Lunn <andrew@lunn.ch>
Cc: Fabio Estevam <festevam@gmail.com>
Cc: Rob Herring <robh@kernel.org>
Cc: linux-kernel@vger.kernel.org
Cc: devicetree@vger.kernel.org
Signed-off-by: Shawn Guo <shawnguo@kernel.org>