irqchip/apple-aic: Add Fast IPI support

The newer AICv2 present in t600x SoCs does not have legacy IPI support
at all. Since t8103 also supports Fast IPIs, implement support for this
first. The legacy IPI code is left as a fallback, so it can be
potentially used by older SoCs in the future.

The vIPI code is shared; only the IPI firing/acking bits change for Fast
IPIs.

Signed-off-by: Hector Martin <marcan@marcan.st>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220309192123.152028-4-marcan@marcan.st