imx31: add watchdog device on litekit board.

This patch adds support for SoC build-in watchdog device on litekit
board.

Signed-off-by: Vladimir Zapolskiy <vzapolskiy@gmail.com>
Cc: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Cc: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>