nds32: fix the missing COBJS-y change

There is a missing in previous
commit 951344b778
(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>

board/AndesTech/adp-ag102/Makefile

@@ -9,7 +9,7 @@ include $(TOPDIR)/config.mk
 
 LIB	= $(obj)lib$(BOARD).o
 
-COBJS	:= adp-ag102.o
+COBJS-y	:= adp-ag102.o
 
 SRCS	:= $(SOBJS-y:.o=.S) $(COBJS-y:.o=.c)
 OBJS	:= $(addprefix $(obj),$(COBJS-y) $(SOBJS-y))