arm64: dts: marvell: clean up armada-7040-db

Instead of duplicating the node hierarchy, reference the nodes by label,
adding labels where necessary.

Drop some trailing or inconsistent white lines while at it.

Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
[Thomas: drop Fixes tag as it is not a bug fix.]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>

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