irqchip/gic-v3: Allow interrupt to be configured as wake-up sources

Although GICv3 doesn't directly offers support for wake-up interrupts
and relies on external HW for this, it shouldn't prevent the driver
for such HW from doing it work.

Let's set the required flags on the irq_chip structures.

Reported-by: Lina Iyer <ilina@codeaurora.org>
Tested-by: Lina Iyer <ilina@codeaurora.org>
Reviewed-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Marc Zyngier <marc.zyngier@arm.com>