provide more common DMA API functions

[Christoph Hellwig]

Since 2009 we have a nice asm-generic header implementing lots of DMA API
functions for architectures using struct dma_map_ops, but unfortunately
it's still missing a lot of APIs that all architectures still have to
duplicate.

This series consolidates the remaining functions, although we still
need arch opt outs for two of them as a few architectures have very
non-standard implementations.