Clarify ranges property

[galak]

From a conversation on the devicetree-spec mailing list, I asked about how the length element in the ranges property is suppose to be used. A few questions asked (quoted response is from David Gibson):

If a child has a reg outside of the length is it not translated via the range?

Yes.

What if the reg size of a child node overlaps the length? Do we translate up to the point the length specifies?

I believe so, yes.

If #size-cells of the node is 0, does that mean no length element?

I think so, yes, although I don't think I've ever seen a 'ranges'
property on a #size-cells = 0 node.

So if we update the spec, we should clarify such concerns w/ranges.