rtc: watchdog support for rtc-m41t80 driver

Add a watchdog driver interface to rtc-m41t80 driver.  This is derived from
works by Alexander Bigga <ab@mycable.de>

Signed-off-by: Atsushi Nemoto <anemo@mba.ocn.ne.jp>
Signed-off-by: Alexander Bigga <ab@mycable.de>
Cc: David Brownell <david-b@pacbell.net>
Acked-by: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>