swiotlb: tidy up includes

SWIOTLB's includes have become a great big mess. Restore some order by
consolidating the random different blocks, sorting alphabetically, and
purging some clearly unnecessary entries - linux/io.h is now included
unconditionally, so need not be duplicated in the restricted DMA pool
case; similarly, linux/io.h subsumes asm/io.h; and by now it's a
mystery why asm/dma.h was ever here at all.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>