scripts: Makefile: also clean subdir dtc

Since commit 93b19653 ("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>