linux-imx

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>