MIPS: Add option to disable software I/O coherency.

Some MIPS controllers have hardware I/O coherency. This patch
detects those and turns off software coherency. A new kernel
command line option also allows the user to manually turn
software coherency on or off.

Signed-off-by: Steven J. Hill <Steven.Hill@imgtec.com>

arch/mips/include/asm/mach-generic/dma-coherence.h

@@ -61,9 +61,8 @@ static inline int plat_device_is_coherent(struct device *dev)
 {
 #ifdef CONFIG_DMA_COHERENT
 	return 1;
-#endif
-#ifdef CONFIG_DMA_NONCOHERENT
-	return 0;
+#else
+	return coherentio;
 #endif
 }