watchdog: intel-mid: add Magic Closure flag

Adding WDIOF_MAGICCLOSE to Intel MID watchdog driver. Once the watchdog
is opened, it makes sense to disable watchdog only if it was gracefully
released.

Signed-off-by: David Cohen <david.a.cohen@linux.intel.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>