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>

arch/arm/boot/dts/imx6sx.dtsi

@@ -1161,6 +1161,8 @@ adc1: adc@02280000 {
 				interrupts = <GIC_SPI 100 IRQ_TYPE_LEVEL_HIGH>;
 				clocks = <&clks IMX6SX_CLK_IPG>;
 				clock-names = "adc";
+				fsl,adck-max-frequency = <30000000>, <40000000>,
+							 <20000000>;
 				status = "disabled";
                         };
 
@@ -1170,6 +1172,8 @@ adc2: adc@02284000 {
 				interrupts = <GIC_SPI 101 IRQ_TYPE_LEVEL_HIGH>;
 				clocks = <&clks IMX6SX_CLK_IPG>;
 				clock-names = "adc";
+				fsl,adck-max-frequency = <30000000>, <40000000>,
+							 <20000000>;
 				status = "disabled";
                         };