watchdog: mpc8xxx: use dynamic memory for device specific data

Instead of relying on global static memory dynamically allocate the
needed data. This has the benefit of some saved bytes if the driver is
not in use and making it possible to bind more than one device (even
though this has no known use case).

Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>