watchdog: imx2_wdt: Disable power down counter on boot

Disable power down counter of the watchdog to avoid system resets. The
watchdog power down counter is set automatically by the chip. If it is
not set to 0 in the driver, the system resets.

Signed-off-by: Markus Pargmann <mpa@pengutronix.de>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>