ARM: dts: imx6qdl-apalis: Pass 'io-channel-cells' to the ADC

The STMPE811 ADC on the Apalis SoM board can be used as a provider
of ADC capabilities to other devices, for example, when a custom
baseboard has an SN74LV4051 analog mux.

Pass the 'io-channel-cells' parameter to indicate such possibility.

While at it, also pass a label for the adc device, so that it can
be referenced in the custom baseboard.

Signed-off-by: Fabio Estevam <festevam@gmail.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>