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/mx6sllevk.h b/include/configs/mx6sllevk.h
index 03c0c88..61c637d 100644
--- a/include/configs/mx6sllevk.h
+++ b/include/configs/mx6sllevk.h
@@ -135,4 +135,5 @@
 #define CONFIG_MXC_USB_PORTSC		(PORT_PTS_UTMI | PORT_PTS_PTW)
 #endif
 
+#include <linux/stringify.h>
 #endif				/* __CONFIG_H */