generic: add common struct for dma map operations

This adds struct dma_map_ops include/linux/dma-mapping.h, which, is
used to handle multiple sets of dma mapping API.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Ingo Molnar <mingo@elte.hu>