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>
diff --git a/Makefile b/Makefile
index 64cbc66..c1dcd85 100644
--- a/Makefile
+++ b/Makefile
@@ -1544,7 +1544,7 @@
 	$(call cmd,rmfiles)
 	@find $(if $(KBUILD_EXTMOD), $(KBUILD_EXTMOD), .) $(RCS_FIND_IGNORE) \
 		\( -name '*.[oas]' -o -name '*.ko' -o -name '.*.cmd' \
-		-o -name '*.ko.*' \
+		-o -name '*.ko.*' -o -name '*.dtb' -o -name '*.dtb.S' \
 		-o -name '*.dwo'  \
 		-o -name '*.su'  \
 		-o -name '.*.d' -o -name '.*.tmp' -o -name '*.mod.c' \