kbuild: make sure we clean up DTB temporary files

Various temporary files used when building DTB files were not suffixed with
.tmp and therefore were not cleaned up by "make clean".

Signed-off-by: Ian Campbell <ian.campbell@citrix.com>
Reviewed-by: Stephen Warren <swarren@nvidia.com>
Tested-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Grant Likely <grant.likely@linaro.org>