ARM: mvebu: Enable Armada 370/XP watchdog in the devicetree

Add the DT nodes to enable watchdog support available in Armada 370
and Armada XP SoCs.

Tested-by: Willy Tarreau <w@1wt.eu>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>