ARM: dts: vfxxx: Add io-channel-cells property for ADC node

This commit adds io-channel-cells property to the ADC node. This
property is required in order for an IIO consumer driver to work.
Especially required for Colibri VF50, as the touchscreen driver
uses ADC channels with the ADC driver based on IIO framework.

Signed-off-by: Sanchayan Maity <maitysanchayan@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>