scripts: Makefile: also clean subdir dtc

Since commit 93b196532254 ("Makefile: Only build dtc if needed"),
the sub directory scripts/dtc is never cleaned.
Adds the directory dtc to subdir to always clean it.

Signed-off-by: Philippe Reynes <philippe.reynes@softathome.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/scripts/Makefile b/scripts/Makefile
index cfe9fef..8731e6c 100644
--- a/scripts/Makefile
+++ b/scripts/Makefile
@@ -9,4 +9,4 @@
 always		:= $(hostprogs-y)
 
 # Let clean descend into subdirs
-subdir-	+= basic kconfig
+subdir-	+= basic kconfig dtc