arm64: dts: marvell: add UART aliases and define stdout-path

This commit adds the necessary UART aliases to the main Armada 7K/8K
.dtsi file, and uses them to define the /chosen/stdout-path property
on the Armada 7040 DB board.

Suggested-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>