Add dma_mmap_coherent() for other archs

Hi,

currently, there is no uniformed way to mmap the pages allocated
via dma_alloc_coherent() properly.  This is a long-standing issue
in the ALSA PCM code, which causes Oops on some non-coherent
architectures.

ARM has already a function dma_mmap_coherent() for that purpose.
This patch series is for adding the same function to other major
architectures to improve the situation (read: not "solve" perfectly :)

I tried to keep the addition as simple and small as possible.
As I couldn't do build tests on some archs, some patches might be
wrong.  A fix patch would be appreciated.

Also, I added ARCH_HAS_DMA_MMAP_COHERENT for each definition so that
the caller can know whether the function is available.  This is
actually more helpful than giving a dummy inline function (at least in
the case of ALSA code) since it can be optimized at the compile time.
Maybe another name would be better, or there is a more clever way.
In anyways, comments and suggestions are greatly appreciated.

The patches can be found on test/dma-fix branch of sound git tree,
too.
  git://git.kernel.org/pub/scm/linux/kernel/git/tiwai/sound-2.6.git test/dma-fix


thanks,

Takashi