ARM: dt: zynq: Add labels to cpu nodes to allow overriding OPPs.

By adding labels to the cpu nodes in the dtsi, a dts that
includes it can change the OPPs by referencing the cpu0
through the label.
Signed-off-by: Moritz Fischer <moritz.fischer@ettus.com>
Reviewed-by: Sören Brinkmann <soren.brinkmann@xilinx.com>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/arm/boot/dts/zynq-7000.dtsi

@@ -19,7 +19,7 @@ cpus {
 		#address-cells = <1>;
 		#size-cells = <0>;
 
-		cpu@0 {
+		cpu0: cpu@0 {
 			compatible = "arm,cortex-a9";
 			device_type = "cpu";
 			reg = <0>;
@@ -33,7 +33,7 @@ cpu@0 {
 			>;
 		};
 
-		cpu@1 {
+		cpu1: cpu@1 {
 			compatible = "arm,cortex-a9";
 			device_type = "cpu";
 			reg = <1>;