dma-mapping: move the dma_coherent flag to struct device

Various architectures support both coherent and non-coherent dma on a
per-device basis.  Move the dma_noncoherent flag from the mips archdata
field to struct device proper to prepare the infrastructure for reuse on
other architectures.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Acked-by: Paul Burton <paul.burton@mips.com>
Acked-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>