Make sure RTC-interrupts are disabled at shutdown.

As the RTC is generally powered by backup power (VDDBU), its interrupts
are not disabled on wake-up, user, watchdog or software reset.  This
could cause troubles on other systems (e.g.  older kernels) if an
interrupt occurs before a handler has been installed at next boot.

Let us be well-behaved and disable them on clean shutdowns at least (as
do the RTT-based rtc-at91sam9 driver).
Signed-off-by: Johan Hovold <jhovold@gmail.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
Cc: Andrew Victor <linux@maxim.org.za>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>