watchdog: mpc8xxx: allow to compile for MPC512x

The MPC5125 processor features a watchdog device that is identical to
the MPC8610 one. So allow to enable the driver for MPC512x kernel
configurations.

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>