blackfin: Add missing macro CONFIG_BFIN_SERIAL

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
diff --git a/include/configs/bf537-minotaur.h b/include/configs/bf537-minotaur.h
index feb9d73..156eeab 100644
--- a/include/configs/bf537-minotaur.h
+++ b/include/configs/bf537-minotaur.h
@@ -136,6 +136,7 @@
 
 #define CONFIG_BAUDRATE		57600
 #define CONFIG_UART_CONSOLE	0
+#define CONFIG_BFIN_SERIAL
 
 #define CONFIG_PANIC_HANG	1
 #define CONFIG_RTC_BFIN		1