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: NMasahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: NArnd Bergmann <arnd@arndb.de>
Signed-off-by: NRob Herring <robh@kernel.org>

Signed-off-by: NRich Felker <dalias@libc.org>