watchdog: mpc8xxx: use the core worker function

The watchdog core includes a worker function which pings the
watchdog until user app starts pinging it and which also
pings it if the HW require more frequent pings.
Use that function instead of the dedicated timer.
In the mean time, we can allow the user to change the timeout.

Then change the timeout module parameter to use seconds and
use the watchdog_init_timeout() core function.

On some HW (eg: the 8xx), SWCRR contains bits unrelated to the
watchdog which have to be preserved upon write.

This driver has nothing preventing the use of the magic close, so
enable it.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>