arm64: dts: marvell: armada-37xx: add nodes to support watchdog

This adds the system controller node for CPU Miscellaneous Registers
(which is needed for the watchdog node) and the watchdog node.

Signed-off-by: Marek Behún <marek.behun@nic.cz>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>