ARM: at91: add usart3 alias to dtsi

Alias was missing for SoC of the at91sam9x5 familly that embed USART3.
Reported-by: Jiri Prchal <jiri.prchal@aksignal.cz>
[b.brezillon@overkiz.com: advised to place changes in at91sam9x5_usart3.dtsi]
Acked-by: Boris BREZILLON <b.brezillon@overkiz.com>
Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>

arch/arm/boot/dts/at91sam9x5_usart3.dtsi

@@ -11,6 +11,10 @@
 #include <dt-bindings/interrupt-controller/irq.h>
 
 / {
+	aliases {
+		serial4 = &usart3;
+	};
+
 	ahb {
 		apb {
 			pinctrl@fffff400 {