ARM: Orion: DTify the watchdog timer.

Add device tree support to the Orion watchdog timer, and enable its
use in the kirkwood devices using device tree.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Sebastian Hesselbarth <sebastian.hesselbarth@googlemail.com>
Tested-by: Simon Baatz <gmbnomis@gmail.com>