Add BAUD_RATE_4800 definition.

git-svn-id: https://rt-thread.googlecode.com/svn/trunk@2538 bbd45198-f89e-11dd-88c7-29a3b14d5316

components/drivers/include/drivers/serial.h

@@ -18,6 +18,7 @@
 
 #include <rtthread.h>
 
+#define BAUD_RATE_4800                  4800
 #define BAUD_RATE_9600                  9600
 #define BAUD_RATE_115200                115200