nds32: fix the missing COBJS-y change

There is a missing in previous
commit 951344b7
(nds32: Convert Makefiles to use COBJS-y style)
will cause compile error.

Signed-off-by: Kuan-Yu Kuo <ken.kuoky@gmail.com>
Cc: Macpaul Lin <macpaul@gmail.com>
Cc: Andes <uboot@andestech.com>
Signed-off-by: Andes <uboot@andestech.com>