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/mips/boot/dts/ni/Makefile

@@ -4,4 +4,3 @@ dtb-$(CONFIG_FIT_IMAGE_FDT_NI169445)	+= 169445.dtb
 obj-					+= dummy.o
 
 always					:= $(dtb-y)
-clean-files				:= *.dtb *.dtb.S