mpc8379erdb: Convert to using DM_SERIAL

  Convert to DM_SERIAL for mpc8379erdb.

Signed-off-by: Sinan Akman <sinan@writeme.com>
diff --git a/include/configs/MPC837XERDB.h b/include/configs/MPC837XERDB.h
index 70b1c39..3967cc2 100644
--- a/include/configs/MPC837XERDB.h
+++ b/include/configs/MPC837XERDB.h
@@ -140,7 +140,9 @@
 /*
  * Serial Port
  */
+#if !CONFIG_IS_ENABLED(DM_SERIAL) && !CONFIG_IS_ENABLED(DM_CLK)
 #define CFG_SYS_NS16550_CLK		get_bus_freq(0)
+#endif
 
 #define CFG_SYS_BAUDRATE_TABLE \
 		{300, 600, 1200, 2400, 4800, 9600, 19200, 38400, 115200}