openrisc: use the generic in-place uncached DMA allocator

Switch openrisc to use the dma-direct allocator and just provide the
hooks for setting memory uncached or cached.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Stafford Horne <shorne@gmail.com>
Reviewed-by: Robin Murphy <robin.murphy@arm.com>