watchdog/mpc8xxx: add a const qualifier

This prepares *of_device_id.data becoming const. Without this change
the following warning would occur:

	drivers/watchdog/mpc8xxx_wdt.c: In function 'mpc8xxx_wdt_probe':
	drivers/watchdog/mpc8xxx_wdt.c:203:11: warning: assignment discards 'const' qualifier from pointer target type [enabled by default]

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
[ukl: split Arnd's patch by driver and add changelog]
Acked-by: Wim Van Sebroeck <wim@iguana.be>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Uwe Kleine-König <u.kleine-koenig@pengutronix.de>