rtc-at91rm9200: add shadow interrupt mask

Add shadow interrupt-mask register which can be used on SoCs where the
actual hardware register is broken.

Note that some care needs to be taken to make sure the shadow mask
corresponds to the actual hardware state.  The added overhead is not an
issue for the non-broken SoCs due to the relatively infrequent
interrupt-mask updates.  We do, however, only use the shadow mask value
as a fall-back when it actually needed as there is still a theoretical
possibility that the mask is incorrect (see the code for details).

Signed-off-by: Johan Hovold <jhovold@gmail.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Douglas Gilbert <dgilbert@interlog.com>
Cc: Jean-Christophe PLAGNIOL-VILLARD <plagnioj@jcrosoft.com>
Cc: Ludovic Desroches <ludovic.desroches@atmel.com>
Cc: Robert Nelson <Robert.Nelson@digikey.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>