ARM: mvebu: dt: add missing alias 'eth3' on Armada XP mv78260

It was correctly set on mv78460 but not on mv78260, resulting in my
OpenBlocks AX3-4 retrieving only 3 of its 4 MAC addresses from the
boot loader.

Cc: Thomas Petazzoni <[e-mail]>
Cc: Gregory CLEMENT <[e-mail]>
Signed-off-by: Willy Tarreau <[e-mail]>
Acked-by: Gregory CLEMENT <[e-mail]>
Signed-off-by: Jason Cooper <[e-mail]>

arch/arm/boot/dts/armada-xp-mv78260.dtsi

···
 		gpio0 = &gpio0;
 		gpio1 = &gpio1;
 		gpio2 = &gpio2;
+		eth3 = &eth3;
 	};
 
 	cpus {
···
 				interrupts = <91>;
 			};
 
-			ethernet@34000 {
+			eth3: ethernet@34000 {
 				compatible = "marvell,armada-370-neta";
 				reg = <0x34000 0x4000>;
 				interrupts = <14>;