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 <thomas.petazzoni@free-electrons.com>
Cc: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Willy Tarreau <w@1wt.eu>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>