watchdog: mpc8xxx: make use of of_device_get_match_data

This function is new in v4.2-rc1 and makes a forward declaration of the
match table superfluous which can so be removed.

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>