blackfin: Add missing macro CONFIG_BFIN_SERIAL

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
diff --git a/include/configs/bf506f-ezkit.h b/include/configs/bf506f-ezkit.h
index 77b6735..5ad3ee7 100644
--- a/include/configs/bf506f-ezkit.h
+++ b/include/configs/bf506f-ezkit.h
@@ -94,6 +94,7 @@
 #define CONFIG_DCACHE_OFF
 #define CONFIG_UART_CONSOLE	0
 #define CONFIG_BAUDRATE 115200
+#define CONFIG_BFIN_SERIAL
 
 #define CONFIG_CMD_MEMORY
 #undef CONFIG_GZIP