staging: xillybus: Open Firmware driver supporting coherent DMA

If the "dma-coherent" property is present in the device tree, the driver will
not perform cache invalidations. This feature significantly improves data
throughput and reduces CPU load.

Signed-off-by: Eli Billauer <eli.billauer@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>