ARM: dts: imx6sl: Add a label for the cpu node

Add a label for the cpu node, so that board devicetree files can
reference to the CPU node.

This is useful for describing a PMIC voltage that supplies the CPU
voltage.

For example:

&cpu0 {
cpu-supply = <&sw1_reg>;

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

arch/arm/boot/dts/imx6sl.dtsi

···
 		#address-cells = <1>;
 		#size-cells = <0>;
 
-		cpu@0 {
+		cpu0: cpu@0 {
 			compatible = "arm,cortex-a9";
 			device_type = "cpu";
 			reg = <0x0>;