ARM: at91: fix at91_sysirq_mask_rtc for sam9x5 SoCs

ARM: at91: fix at91_sysirq_mask_rtc for sam9x5 SoCs

commit 9dcc87fec8947308e0111c65dcd881e6aa5b1673 upstream.

sam9x5 SoCs have the following errata:
 "RTC: Interrupt Mask Register cannot be used
  Interrupt Mask Register read always returns 0."

Hence we should not rely on what IMR claims about already masked IRQs
and just disable all IRQs.

Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Reported-by: Bryan Evenson <bevenson@melinkcorp.com>
Reviewed-by: Johan Hovold <johan@hovold.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Cc: Bryan Evenson <bevenson@melinkcorp.com>
Cc: Andrew Victor <linux@maxim.org.za>
Cc: Jean-Christophe Plagniol-Villard <plagnioj@jcrosoft.com>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: Mark Roszko <mark.roszko@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Jiri Slaby <jslaby@suse.cz>