[MIPS] Unify dma-{coherent,noncoherent.ip27,ip32}

Platforms will now have to supply a function dma_device_is_coherent which
returns if a particular device participates in the coherence domain.  For
most platforms this function will always return 0 or 1.

Signed-off-by: Ralf Baechle <ralf@linux-mips.org>
diff --git a/include/asm-mips/mach-generic/kmalloc.h b/include/asm-mips/mach-generic/kmalloc.h
index 410ab5f..b8e6deb 100644
--- a/include/asm-mips/mach-generic/kmalloc.h
+++ b/include/asm-mips/mach-generic/kmalloc.h
@@ -5,6 +5,7 @@
 #ifndef CONFIG_DMA_COHERENT
 /*
  * Total overkill for most systems but need as a safe default.
+ * Set this one if any device in the system might do non-coherent DMA.
  */
 #define ARCH_KMALLOC_MINALIGN	128
 #endif