nios2: move core-y in arch/nios2/Makefile to arch/nios2/Kbuild

Use obj-y to clean up Makefile.
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>

arch/nios2/Kbuild

 # SPDX-License-Identifier: GPL-2.0-only
 
+obj-y += kernel/ mm/ platform/ boot/dts/
+
 # for cleaning
 subdir- += boot

arch/nios2/Makefile

@@ -39,8 +39,6 @@ KBUILD_CFLAGS += -G 0
 
 head-y		:= arch/nios2/kernel/head.o
 libs-y		+= arch/nios2/lib/ $(LIBGCC)
-core-y		+= arch/nios2/kernel/ arch/nios2/mm/
-core-y		+= arch/nios2/platform/
 
 INSTALL_PATH ?= /tftpboot
 nios2-boot := arch/$(ARCH)/boot
@@ -48,8 +46,6 @@ BOOT_TARGETS = vmImage zImage
 PHONY += $(BOOT_TARGETS) install
 KBUILD_IMAGE := $(nios2-boot)/vmImage
 
-core-y	+= $(nios2-boot)/dts/
-
 all: vmImage
 
 $(BOOT_TARGETS): vmlinux