drivers/rtc/rtc-at91rm9200.c: use a variable for storing IMR

On some revisions of AT91 SoCs, the RTC IMR register is not working.
Instead of elaborating a workaround for that specific SoC or IP version,
we simply use a software variable to store the Interrupt Mask Register
and modify it for each enabling/disabling of an interrupt.  The overhead
of this is negligible anyway.

The interrupt mask register (IMR) for the RTC is broken on the AT91SAM9x5
sub-family of SoCs (good overview of the members here:
http://www.eewiki.net/display/linuxonarm/AT91SAM9x5 ).  The "user visible
effect" is the RTC doesn't work.

That sub-family is less than two years old and only has devicetree (DT)
support and came online circa lk 3.7 .  The dust is yet to settle on the
DT stuff at least for AT91 SoCs (translation: lots of stuff is still
broken, so much that it is hard to know where to start).

The fix in the patch is pretty simple: just shadow the silicon IMR
register with a variable in the driver.  Some older SoCs (pre-DT) use the
the rtc-at91rm9200 driver (e.g.  obviously the AT91RM9200) and they should
not be impacted by the change.  There shouldn't be a large volume of
interrupts associated with a RTC.

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Reported-by: Douglas Gilbert <dgilbert@interlog.com>
Cc: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Cc: Ludovic Desroches <ludovic.desroches@atmel.com>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>