arm: mvebu: add uartX labels for Armada SoC serial nodes

This patch adds uartX labels for Armada SoC serial nodes. This is
a preliminary work to be able to easily reference the serial lines
in Device Tree files. One expected use is when providing stdout-path
property for barebox.

Reviewed-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Arnaud Ebalard <arno@natisbad.org>
Link: https://lkml.kernel.org/r/0683d1a823fe9b75849f3dafcf1cf6ee291cdca6.1416613429.git.arno@natisbad.org


Signed-off-by: Jason Cooper <jason@lakedaemon.net>