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.

[Based on linux (400b6a0cbef55d1ae32808eaa1ef1c28820bf6ac)]
Signed-off-by: Moritz Fischer <moritz.fischer@ettus.com>
Cc: Michal Simek <michal.simek@xilinx.com>
Cc: u-boot@lists.denx.de
Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/arm/dts/zynq-7000.dtsi

@@ -16,7 +16,7 @@
 		#address-cells = <1>;
 		#size-cells = <0>;
 
-		cpu@0 {
+		cpu0: cpu@0 {
 			compatible = "arm,cortex-a9";
 			device_type = "cpu";
 			reg = <0>;
@@ -30,7 +30,7 @@
 			>;
 		};
 
-		cpu@1 {
+		cpu1: cpu@1 {
 			compatible = "arm,cortex-a9";
 			device_type = "cpu";
 			reg = <1>;