ARM: dts: imx: Add an cpu0 label for imx6dl devices

Adding the label cpu0 allows the adjustment of cpu-parameters
by reference in overlaying dtsi files in the same way as it
is possible for imx6q devices.

Signed-off-by: Jan Tuerk <[e-mail]>
Reviewed-by: Andreas Färber <[e-mail]>
Signed-off-by: Shawn Guo <[e-mail]>

arch/arm/boot/dts/imx6dl.dtsi

···
 		#address-cells = <1>;
 		#size-cells = <0>;
 
-		cpu@0 {
+		cpu0: cpu@0 {
 			compatible = "arm,cortex-a9";
 			device_type = "cpu";
 			reg = <0>;