centralize SWIOTLB config symbol and misc other cleanups V3

[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.

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.