ARM: zynq: DT: Add reference to bus node

For adding OCM memory in platform DTS is necessary to have reference to
amba bus.

Signed-off-by: Michal Simek <michal.simek@xilinx.com>

arch/arm/dts/zynq-7000.dtsi

@@ -53,7 +53,7 @@
 		regulator-always-on;
 	};
 
-	amba {
+	amba: amba {
 		compatible = "simple-bus";
 		#address-cells = <1>;
 		#size-cells = <1>;