watchdog: imx2_wdt: disable watchdog timer during low power mode

We should set watchdog timer to be disabled in low power mode,
as there is no service running in background, otherwise, system
will reset unexpected.

Signed-off-by: Anson Huang <b20788@freescale.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>