riscv: nx25: dts: Add AE250 dts to support RISC-V

AE250 is the Soc using NX25 cpu core base on RISC-V arch.
Details please see the doc/README.ae250.

Signed-off-by: Rick Chen <rick@andestech.com>
Signed-off-by: Rick Chen <rickchen36@gmail.com>
Signed-off-by: Greentime Hu <green.hu@gmail.com>
diff --git a/arch/riscv/dts/Makefile b/arch/riscv/dts/Makefile
new file mode 100644
index 0000000..718b99f
--- /dev/null
+++ b/arch/riscv/dts/Makefile
@@ -0,0 +1,14 @@
+#
+# SPDX-License-Identifier:	GPL-2.0+
+#
+
+dtb-$(CONFIG_TARGET_NX25_AE250) += ae250.dtb
+targets += $(dtb-y)
+
+DTC_FLAGS += -R 4 -p 0x1000
+
+PHONY += dtbs
+dtbs: $(addprefix $(obj)/, $(dtb-y))
+	@:
+
+clean-files := *.dtb