watchdog: imx2_wdt: expose module alias for loading from device-tree

Enable auto loading by udev when imx2_wdt is compiled as a module.

Signed-off-by: Niels de Vos <ndevos@redhat.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>