ppc: amcc: Omit unneeded ns16550 CONFIG if using driver model

This comes from the device tree or a call to get_uart_clock().

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/configs/amcc-common.h b/include/configs/amcc-common.h
index 2aea899..73e1b0a 100644
--- a/include/configs/amcc-common.h
+++ b/include/configs/amcc-common.h
@@ -20,8 +20,10 @@
  */
 #define CONFIG_SYS_NS16550
 #define CONFIG_SYS_NS16550_SERIAL
+#ifndef CONFIG_DM_SERIAL
 #define CONFIG_SYS_NS16550_REG_SIZE	1
 #define CONFIG_SYS_NS16550_CLK		get_serial_clock()
+#endif
 #define CONFIG_BAUDRATE		115200
 #define CONFIG_SYS_BAUDRATE_TABLE  \
     {300, 600, 1200, 2400, 4800, 9600, 19200, 38400, 57600, 115200, 230400}