centralize SWIOTLB config symbol and misc other cleanups V3

[Christoph Hellwig]

Any more comments?  Especially from the x86, mips and powerpc arch
maintainers?  I'd like to merge this in a few days as various other
patches depend on it.

On Wed, Apr 25, 2018 at 07:15:26AM +0200, Christoph Hellwig wrote:
> Hi all,
> 
> this seris aims for a single defintion of the Kconfig symbol.  To get
> there various cleanups, mostly about config symbols are included as well.
> 
> Changes since V2:
>  - swiotlb doesn't need the dma_length field by itself, so don't select it
>  - don't offer a user visible SWIOTLB choice
> 
> Chages since V1:
>  - fixed a incorrect Reviewed-by that should be a Signed-off-by.
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