RTC: add periodic irq support to rtc-cmos

Adds support for periodic irq enabling in rtc-cmos.  This could be used by
the ALSA driver and is already being tested with the zaptel ztdummy module.

Signed-off-by: Alessandro Zummo <a.zummo@towertech.it>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>