Blackfin arch: define BASE_BAUD like everyone else

define BASE_BAUD like everyone else, althought it is only used by 8250
Signed-off-by: NMike Frysinger <vapier.adi@gmail.com>
Signed-off-by: NBryan Wu <cooloney@kernel.org>

arch/blackfin/include/asm/serial.h

@@ -3,3 +3,4 @@
  */
 
 #define SERIAL_EXTRA_IRQ_FLAGS IRQF_TRIGGER_HIGH
+#define BASE_BAUD (1843200 / 16)