irqchip/apple-aic: Support multiple dies

Multi-die support in AICv2 uses several sets of IRQ registers. Introduce
a die count and compute the register group offset based on the die ID
field of the hwirq number, as reported by the hardware.

Signed-off-by: Hector Martin <marcan@marcan.st>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220309192123.152028-7-marcan@marcan.st