dt-bindings: apple,aic: Add affinity description for per-cpu pseudo-interrupts

Some of the FIQ per-cpu pseudo-interrupts are better described with
a specific affinity, the most obvious candidate being the CPU PMUs.

Augment the AIC binding to be able to specify that affinity in the
interrupt controller node.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Marc Zyngier <maz@kernel.org>