watchdog: mei_wdt: add activation debugfs entry

Add entry for displaying whether the device has activated or
deactivated watchdog fw application.

cat <debugfs>/mei_wdt/activation
activated | deactivated

Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Tomas Winkler <tomas.winkler@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>