watchdog: mpc8xxx_wdt: MPC8xx is HW enabled

MPC8xx watchdog is enabled at startup by HW.
If the bootloader disables it, it cannot be reenabled.

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