dt-bindings: interrupt-controller: apple,aic: Add apple,aic2 support

dt-bindings: interrupt-controller: apple,aic: Add apple,aic2 support

This new incompatible revision of the AIC peripheral introduces
multi-die support. To handle that, we introduce an optional
4-argument interrupt-cells form.

Also add an apple,event-reg property to specify the offset of the event
register. Inexplicably, the capability registers allow us to compute
other register offsets, but not this one. This allows us to keep
forward-compatibility with future SoCs that will likely implement
different die counts, thus shifting the event register. Apple do the
same thing in their device tree...

Signed-off-by: Hector Martin <marcan@marcan.st>