dt-bindings: interrupt-controller: apple,aic: Add power-domains property

This will bind to the PMGR pwrstate nodes that control power/clock
gating to SoC blocks. The AIC driver doesn't do runtime-pm and likely
never will (since it is system-critical), but it makes sense to describe
the power domain relationship the devicetree properly.

Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Hector Martin <marcan@marcan.st>