irqchip/apple-aic: Dynamically compute register offsets

This allows us to support AIC variants with different numbers of IRQs
based on capability registers.

Signed-off-by: Hector Martin <marcan@marcan.st>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220309192123.152028-6-marcan@marcan.st