watchdog: iTCO_wdt: Use allocated data structures

Allocate private data and the watchdog device to avoid having
to clear it on remove and to enable subsequent simplifications.
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Mika Westerberg <mika.westerberg@linux.intel.com>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>