irqchip/exiu: Fix acknowledgment of edge triggered interrupts

Currently the EXIU uses the fasteoi interrupt flow that is configured by
it's parent (irq-gic-v3.c). With this flow the only chance to clear the
interrupt request happens during .irq_eoi() and (obviously) this happens
after the interrupt handler has run. EXIU requires edge triggered
interrupts to be acked prior to interrupt handling. Without this we
risk incorrect interrupt dismissal when a new interrupt is delivered
after the handler reads and acknowledges the peripheral but before the
irq_eoi() takes place.

Fix this by clearing the interrupt request from .irq_ack() if we are
configured for edge triggered interrupts. This requires adopting the
fasteoi-ack flow instead of the fasteoi to ensure the ack gets called.

These changes have been tested using the power button on a
Developerbox/SC2A11 combined with some hackery in gpio-keys so I can
play with the different trigger mode [and an mdelay(500) so I can
can check what happens on a double click in both modes].

Fixes: 706cffc1 ("irqchip/exiu: Add support for Socionext Synquacer EXIU controller")
Signed-off-by: Daniel Thompson <daniel.thompson@linaro.org>
Reviewed-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220503134541.2566457-1-daniel.thompson@linaro.org