h8300: move core-y in arch/h8300/Makefile to arch/h8300/Kbuild

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

arch/h8300/Kbuild

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

arch/h8300/Makefile

@@ -30,9 +30,6 @@ ifeq ($(CROSS_COMPILE),)
 CROSS_COMPILE := $(call cc-cross-prefix, h8300-unknown-linux- h8300-linux-)
 endif
 
-core-y	+= arch/$(ARCH)/kernel/ arch/$(ARCH)/mm/
-core-y	+= arch/$(ARCH)/boot/dts/
-
 libs-y	+= arch/$(ARCH)/lib/
 
 boot := arch/h8300/boot