kbuild: clean up *.dtb and *.dtb.S patterns from top-level Makefile

We need to add "clean-files" in Makfiles to clean up DT blobs, but we
often miss to do so.

Since there are no source files that end with .dtb or .dtb.S, so we
can clean-up those files from the top-level Makefile.

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Rob Herring <robh@kernel.org>

arch/c6x/boot/dts/Makefile

@@ -16,5 +16,3 @@ $(obj)/builtin.dtb: $(obj)/$(DTB).dtb
 	$(call if_changed,cp)
 
 $(obj)/linked_dtb.o: $(obj)/builtin.dtb
-
-clean-files := *.dtb