arm64: dts: hikey: add label properties to UARTs

Add label properties to provide a way to identify UARTs based on their
board or connector name. This follows naming convention in 96boards CE
spec. Ports without external connections are not labelled.
Signed-off-by: Rob Herring <robh@kernel.org>
Signed-off-by: Olof Johansson <olof@lixom.net>

arch/arm64/boot/dts/hisilicon/hi6220-hikey.dts

@@ -32,3 +32,10 @@ memory@0 {
 		reg = <0x0 0x0 0x0 0x40000000>;
 	};
 };
+
+&uart2 {
+	label = "LS-UART0";
+};
+&uart3 {
+	label = "LS-UART1";
+};