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/dart_6ul.h b/include/configs/dart_6ul.h
index 15d6884..d933f79 100644
--- a/include/configs/dart_6ul.h
+++ b/include/configs/dart_6ul.h
@@ -7,6 +7,7 @@
 #define __DART_6UL_H
 
 #include <linux/sizes.h>
+#include <linux/stringify.h>
 #include "mx6_common.h"
 
 /* SPL options */