MIPS: Allow MAX_DMA32_PFN to be overridden.

DMA mapping may reduce the usable physical address range usable for
32-bit DMA.
Signed-off-by: David Daney <ddaney@caviumnetworks.com>
Patchwork: http://patchwork.linux-mips.org/patch/1633/Signed-off-by: Ralf Baechle <ralf@linux-mips.org>

arch/mips/include/asm/dma.h

@@ -91,7 +91,10 @@
 #define MAX_DMA_ADDRESS		(PAGE_OFFSET + 0x01000000)
 #endif
 #define MAX_DMA_PFN		PFN_DOWN(virt_to_phys((void *)MAX_DMA_ADDRESS))
+
+#ifndef MAX_DMA32_PFN
 #define MAX_DMA32_PFN		(1UL << (32 - PAGE_SHIFT))
+#endif
 
 /* 8237 DMA controllers */
 #define IO_DMA1_BASE	0x00	/* 8 bit slave DMA, channels 0..3 */