rtc: rtc-at91rm9200: fix infinite wait for ACKUPD irq

rtc: rtc-at91rm9200: fix infinite wait for ACKUPD irq

The rtc user must wait at least 1 sec between each time/calandar update
(see atmel's datasheet chapter "Updating Time/Calendar").

Use the 1Hz interrupt to update the at91_rtc_upd_rdy flag and wait for
the at91_rtc_wait_upd_rdy event if the rtc is not ready.

This patch fixes a deadlock in an uninterruptible wait when the RTC is
updated more than once every second.  AFAICT the bug is here from the
beginning, but I think we should at least backport this fix to 3.10 and
the following longterm and stable releases.

Signed-off-by: Boris BREZILLON <[email protected]>
Reported-by: Bryan Evenson <[email protected]>
Tested-by: Bryan Evenson <[email protected]>
Cc: Andrew Victor <[email protected]>
Cc: Nicolas Ferre <[email protected]>
Cc: Jean-Christophe Plagniol-Villard <[email protected]>
Cc: Alessandro Zummo <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>