openrisc: provide dma_map_ops

This switches OpenRISC over to fully using the generic dma-mapping
framework.  This was almost already the case as the architecture's
implementation was essentially a copy of the generic header.

This also brings this architecture in line with the recent changes
to dma_map_ops (adding attributes to ops->alloc).

Signed-off-by: Jonas Bonn <jonas@southpole.se>