arm64: dts: imx8-ss-dma: add io-channel-cells to adc nodes

This commit adds io-channel-cells property to the ADC nodes. This
property is required in order for an IIO consumer driver to work.
Especially required for Apalis iMX8 QM, as the touchscreen driver
uses ADC channels with the ADC driver based on IIO framework.

Signed-off-by: Philippe Schenker <philippe.schenker@toradex.com>
Signed-off-by: Max Krummenacher <max.krummenacher@toradex.com>
Signed-off-by: Marcel Ziswiler <marcel.ziswiler@toradex.com>
Acked-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>