dma-direct: add support for allocation from ZONE_DMA and ZONE_DMA32

This allows to dip into zones for lower memory if they are available.
If one of the zones is not available the corresponding GFP_* flag
will evaluate to 0 so they won't change anything.  We provide an
arch tunable for those architectures that do not use GFP_DMA for
the lowest 24-bits, given that there are a few.

Roughly based on the x86 code.

Signed-off-by: Christoph Hellwig <hch@lst.de>