ARM: dts: imx6sx: Pass the 'adck-max-frequency' property

Specify the 'adck-max-frequency' property in the adc nodes.

According to Documentation/devicetree/bindings/iio/adc/vf610-adc.txt
this is a recommended property.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Fugang Duan <B38611@freescale.com>
Signed-off-by: Shawn Guo <shawnguo@kernel.org>