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 <[e-mail]>
Signed-off-by: Shawn Guo <[e-mail]>

arch/arm/boot/dts/imx6qdl-apalis.dtsi

···
 			st,touch-det-delay = <5>;
 		};
 
-		stmpe-adc {
+		stmpe_adc: stmpe-adc {
 			compatible = "st,stmpe-adc";
 			/* forbid to use ADC channels 3-0 (touch) */
 			st,norequest-mask = <0x0F>;
+			#io-channel-cells = <1>;
 		};
 	};
 };