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>
diff --git a/arch/riscv/Kbuild b/arch/riscv/Kbuild
new file mode 100644
index 0000000..d1d0aa7
--- /dev/null
+++ b/arch/riscv/Kbuild
@@ -0,0 +1,3 @@
+# SPDX-License-Identifier: GPL-2.0-only
+
+obj-y += kernel/ mm/ net/