ARM: dts: mvebu: armada-39x: enable rtc for all Armada-39x SoCs

Despite that FS states that rtc is present only in A395 and A398 and not in
A390, the rtc is working with A390.

Signed-off-by: Grzegorz Jaszczyk <jaz@semihalf.com>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>