nds32: remove unneeded clean-files for DTB

These patterns are cleaned-up by the top-level Makefile

Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Acked-by: Greentime Hu <green.hu@gmail.com>
Signed-off-by: Greentime Hu <green.hu@gmail.com>