[ARM] Kirkwood: Add the watchdog timer as a platform device.

The Kirkwood architecture uses the same watchdog device as the Orion
architecture. This patch adds orion5x_wdt as a platform device for
Kirkwood.

Signed-off-by: Thomas Reitmayr <treitmayr@devbase.at>
Tested-by: Martin Michlmayr <tbm@cyrius.com>
Signed-off-by: Nicolas Pitre <nico@marvell.com>