Currently x86_32, sh and cris-v32 provide per-device coherent dma
memory allocator.

However their implementation is nearly identical. Refactor out
common code to be reused by them.
Signed-off-by: NDmitry Baryshkov <dbaryshkov@gmail.com>
Signed-off-by: NIngo Molnar <mingo@elte.hu>