obj-* needs a trailing slash for a directory, but subdir-* does not
because it already implies a directory.

Also, change subdir-y to subdir- to ensure this is effective only
for cleaning targets.

This makes the cleaning log consistent. (no trailing slash)

Before:

  $ make clean
  CLEAN   Documentation/devicetree/bindings/

After:

  $ make clean
  CLEAN   Documentation/devicetree/bindings
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>