PCI: apple: Change MSI handling to handle 4-cell AIC fwspec form

AIC2 changes the IRQ fwspec to add a cell. Always use the second-to-last
cell for the MSI handling, so it will work for both AIC1 and AIC2 devices.

Signed-off-by: Hector Martin <marcan@marcan.st>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220309192123.152028-2-marcan@marcan.st