kbuild: Disallow DTB overlays to built from .dts named source files

As a follow up to the series allowing DTB overlays to built from .dtso
files. Now that all overlays have been renamed, remove the ability to
build from overlays from .dts files to prevent any files with the old
name from accidental being added.

Signed-off-by: Andrew Davis <afd@ti.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Andy Shevchenko <andriy.shevchenko@intel.com>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>