watchdog: mpc8xxx: use dev_xxxx() instead of pr_xxxx()

mpc8xxx watchdog driver is a platform device drivers, it is
therefore possible to use dev_xxx() messaging rather than pr_xxx()

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@linux-watchdog.org>