irqchip/apple-aic: Switch to irq_domain_create_tree and sparse hwirqs

This allows us to directly use the hardware event number as the hwirq
number. Since IRQ events have bit 16 set (type=1), FIQs now move to
starting at hwirq number 0.

This will become more important once multi-die support is introduced in
a later commit.

Signed-off-by: Hector Martin <marcan@marcan.st>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20220309192123.152028-5-marcan@marcan.st