centralize SWIOTLB config symbol and misc other cleanups V2

[Christoph Hellwig]

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.

Chances since V2 are a fixed s/Reviewed/Signed-Off/ for me, and a few
reviewed-by tags.  I'd like to start merging this into the dma-mapping
tree rather sooner than later given that quite a bit of material for
this series depends on it.