riscv: dts: fix CONFIG_DEFAULT_DEVICE_TREE failure

It occurs since commit 27cb7300
("Ensure device tree DTS is compiled").

More details can refer to
89c2b5c0
ARM: fix arch/arm/dts/Makefile

Signed-off-by: Rick Chen <rick@andestech.com>
Cc: Greentime Hu <greentime@andestech.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>