arm64: dts: marvell: Clean up armada-3720-db

Instead of duplicating the SoC's node hierarchy, including a bus node
named "internal-regs", reference the actually desired nodes by label,
like Berlin already does. Add labels where necessary.

Drop an inconsistent white line while at it.
Cc: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
[gregory.clement@free-electrons.com: drop Fixes tag as it is not a bug fix.]