ARM: sun6i: Add missing serial aliases

Some UART aliases have been defined, but not all of them. Add the remaining
ones to be consistent and to ease the parsing of the DT by the bootloaders.
Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>

arch/arm/boot/dts/sun6i-a31.dtsi

@@ -16,6 +16,16 @@
 / {
 	interrupt-parent = <&gic>;
 
+	aliases {
+		serial0 = &uart0;
+		serial1 = &uart1;
+		serial2 = &uart2;
+		serial3 = &uart3;
+		serial4 = &uart4;
+		serial5 = &uart5;
+	};
+
+
 	cpus {
 		#address-cells = <1>;
 		#size-cells = <0>;