rtc: stmp3xxx: add wdt-accessor function

This RTC also includes a watchdog timer. Provide an accessor function
for setting the watchdog timeout value which will be picked up by a
watchdog driver. Also register the platform_device for the watchdog here
to get the boot-time dependencies right.

Signed-off-by: Wolfram Sang <w.sang@pengutronix.de>
Acked-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>