genirq: Add fasteoi IPI flow

For irqchips using the fasteoi flow, IPIs are a bit special.
They need to be EOI'd early (before calling the handler), as
funny things may happen in the handler (they do not necessarily
behave like a normal interrupt).

Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>