add model id to milkv's device tree

build/boards/cv180x/cv1800b_milkv_duo_sd/dts_riscv/cv1800b_milkv_duo_sd.dts

@@ -26,6 +26,6 @@
 };
 
 / {
-
+	model = "Milk-V Duo";
 };
 

build/boards/cv181x/cv1812cp_milkv_duo256m_sd/dts_riscv/cv1812cp_milkv_duo256m_sd.dts

@@ -31,6 +31,6 @@
 };
 
 / {
-
+	model = "Milk-V Duo256M";
 };
 

build/boards/cv181x/cv1813h_milkv_duos_emmc/dts_riscv/cv1813h_milkv_duos_emmc.dts

@@ -46,6 +46,6 @@
 };
 
 / {
-
+	model = "Milk-V DuoS";
 };
 

build/boards/cv181x/cv1813h_milkv_duos_sd/dts_riscv/cv1813h_milkv_duos_sd.dts

@@ -46,6 +46,6 @@
 };
 
 / {
-
+	model = "Milk-V DuoS";
 };