ARM: mvebu: add serial port aliases on Armada 370/375/38x/XP

This commit adds 'serialX' aliases for the various serial ports on
Armada 370, 375, 38x and XP platforms. It will allow the usage of the
stdout-path property.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>