riscv: dts: fix CONFIG_DEFAULT_DEVICE_TREE failure

It occurs since commit 27cb7300ffda
("Ensure device tree DTS is compiled").

More details can refer to
89c2b5c02049aea746b1edee0b4e1d8519dec2f4
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>
diff --git a/arch/riscv/dts/Makefile b/arch/riscv/dts/Makefile
index b400def..f9cd606 100644
--- a/arch/riscv/dts/Makefile
+++ b/arch/riscv/dts/Makefile
@@ -1,5 +1,7 @@
 # SPDX-License-Identifier: GPL-2.0+
 
+dtb-$(CONFIG_TARGET_AX25_AE350) += ae350_32.dtb ae350_64.dtb
+
 targets += $(dtb-y)
 
 DTC_FLAGS += -R 4 -p 0x1000