watchdog: make imx2_wdt report boot status correctly

Ioctl WDIOC_GETBOOTSTATUS is supposed to return some information
on why the system did (re)boot recently, value WDIOF_CARDRESET
being used to indicate watchdog induced reboot.

Up to now, imx2_wdt did not provide a value here, always returning
zero to indicate normal boot.

Do evaluate the IMX Watchdog Reset Status Register and
produce WDIOF_CARDRESET with WDIOC_GETBOOTSTATUS in case
of a watchdog induced reset.

Signed-off-by: Oskar Schirmer <oskar@scara.com>
Acked-by: Wolfram Sang <w.sang@pengutronix.de>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>