imx3: Add watchdog platform device support

This patch adds support for build-in watchdog device found on
Freescale imx31 and imx35 SoCs.

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