[PATCH] ARM: at91: fix rtc irq mask for sam9x5 SoCs

[Mark Roszko]

Atmel actually has this issue in the Errata of the SAM9G25 and SAM9G35
datasheets which might be worth referencing in the description?

>49.7.1 RTC: Interrupt Mask Register cannot be used
>Interrupt Mask Register read always returns 0.