RTC: use fallback IRQ if PNP tables don't provide one

Intel Macs (and possibly other machines) provide a PNP entry for the RTC,
but provide no IRQ.  As a result the rtc-cmos driver doesn't allow wakeup
alarms.  If the RTC is located at the legacy ioport range, assume that it's
on IRQ 8 unless the tables say otherwise.

Signed-off-by: Matthew Garrett <mjg59@srcf.ucam.org>
Cc: Matthieu CASTET <castet.matthieu@free.fr>
Signed-off-by: David Brownell <dbrownell@users.sourceforge.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>