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

The whole Armada 39x SoC family of processors has watchdog which can be
supported with existing driver.

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