arch/powerpc Kconfig cleanup

[Li Yang-r58472]

> arch/powerpc/Kconfig has become something of a mess and platform
support
> is scattered between it and various arch/powerpc/platforms/*/Kconfig
> files.  I've got some ideas on some simple clean up that will
introduce a
> arch/powerpc/platforms/Kconfig and pull out platform config
information
> from arch/powerpc/Kconfig.
> 
> Beyond general cleanup, there are several config options that should
> exist under the "Platform support" menu but end up getting duplicated
> across sub-architectures (QE, CPM2, etc.)
> 
> By introducing a platforms/Kconfig we will have a place to put those
> config options so we don't have to duplicate them.

Sounds good to me.

- Leo