watchdog: mpc8xxx: Remove unneeded linux/miscdevice.h include

watchdog/mpc8xxx_wdt.c does not use any miscdevice so this patch remove
this unnecessary inclusion.

Signed-off-by: Corentin Labbe <clabbe.montjoie@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Guenter Roeck <linux@roeck-us.net>