irqchip: atmel-aic: Implement RTC irq fixup

Provide an implementation to fix RTC irqs before enabling the irqchip.

This was previously done in arch/arm/mach-at91/sysirq_mask.c but as we're
trying to use standard implementation (IRQCHIP_DECLARE and automatic call
of irqchip_init within arch/arm/kernel/irq.c) we need to do those fixups
in the irqchip driver.

Signed-off-by: Boris BREZILLON <boris.brezillon@free-electrons.com>
Link: https://lkml.kernel.org/r/1405016741-2407-3-git-send-email-boris.brezillon@free-electrons.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>