riscv: add arch/riscv/Kbuild

Use the standard obj-y form to specify the sub-directories under
arch/riscv/. No functional change intended.
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>

arch/riscv/Kbuild

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

arch/riscv/Makefile

@@ -72,7 +72,7 @@ KBUILD_IMAGE	:= $(boot)/Image.gz
 
 head-y := arch/riscv/kernel/head.o
 
-core-y += arch/riscv/kernel/ arch/riscv/mm/ arch/riscv/net/
+core-y += arch/riscv/
 
 libs-y += arch/riscv/lib/