common: Drop linux/stringify.h from common header

Move this uncommon header out of the common header.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/include/configs/mx6cuboxi.h b/include/configs/mx6cuboxi.h
index e10e768..2ccf44e 100644
--- a/include/configs/mx6cuboxi.h
+++ b/include/configs/mx6cuboxi.h
@@ -7,6 +7,8 @@
 #ifndef __MX6CUBOXI_CONFIG_H
 #define __MX6CUBOXI_CONFIG_H
 
+#include <linux/stringify.h>
+
 #include "mx6_common.h"
 
 #include "imx6_spl.h"