[snd] looking for layout-ids

[Benjamin Herrenschmidt]

> Once I have all these, I will, along with submitting snd-aoa [2], submit
> a patch to snd-powermac that makes it refuse loading in presence of a
> layout-id property as a first step to migrate to snd-aoa.
> 
> At the same time, I'll probably make i2sbus refuse attaching to a device
> unless there's a layout-id property so that it doesn't claim devices aoa
> doesn't handle yet.

I think we need to be careful about a couple other things:

 - We need non-pmf GPIO. Some layout-id based machines do need that,
like the mac mini
 - I'm tempted to rename soundbus/i2sbus to snd-aoa-soundbus and
snd-aoa-i2sbus (at least the module names) for now. 

Ben.