irqchip/atmel-aic5: Add support for sam9x60 rtt fixup

Add support for SAM9X60 RTT fixup.

Signed-off-by: Claudiu Beznea <claudiu.beznea@microchip.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Reviewed-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/1591779936-18577-2-git-send-email-claudiu.beznea@microchip.com