irqchip/atmel-aic5: Handle suspend to RAM

On sama5d2, VDD core may be cut while suspending to RAM. This means the
AIC5 registers content is lost. Restore it at resume time.

Signed-off-by: Alexandre Belloni <alexandre.belloni@free-electrons.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>