[PATCH] powerpc: document new interrupt-array property

[Yoder Stuart-B08248]

> > we need something like interrupt-array.
> 
> Again, the "interrupt-array" helps only this one specific
> case, and not the general case *at all*.
> 

Yes-- The interrupt-array helps only this specific case. But
the question is, will this specific case be common in the
future?

If this is a one-off situation it doesn't warrant a new
general property.  However, if it becomes common enough
a simpler representation will benefit developers.  There are
developers who have a hard enough time figuring out the
interrupt-map scheme, much less a logical interrupt nexus
node.

Stuart