Blackfin arch: dma add some API and cleanup bf54x DMA definition

Signed-off-by: Bryan Wu <bryan.wu@analog.com>

diff --git a/include/asm-blackfin/mach-bf548/dma.h b/include/asm-blackfin/mach-bf548/dma.h
index 14cb10c..4d97d3a 100644
--- a/include/asm-blackfin/mach-bf548/dma.h
+++ b/include/asm-blackfin/mach-bf548/dma.h
@@ -70,5 +70,5 @@
 #define MAX_BLACKFIN_DMA_CHANNEL 32
 
 extern int channel2irq(unsigned int channel);
-extern struct dma_register *base_addr[];
+extern struct dma_register *base_addr[MAX_BLACKFIN_DMA_CHANNEL];
 #endif