DMA-related cleanups for parisc v2

[Christoph Hellwig]

Hi James and Helge,

this series has a couple DMA-related cleanups for parisc.  The main aim
is to move anything not required by drivers out of <asm/dma-mapping.h>,
but I noticed a few related bits and cleaned them up as well.

Changes since v1:
 - include <linux/pci.h> in drivers/parisc/iommu.h to avoid build
   failures with some configs