riscv: ae350: dts: Add missing "u-boot, dm-spl" for SPL config

At present the AE350 SPL defconfig is using OF_PRIOR_STAGE. The
intention was to use gdb to load device tree before running U-Boot
SPL/proper from RAM. When we switch to OF_SEPARATE we will have to
use our own DT but without "u-boot,dm-spl" in several essential
nodes, SPL does not boot.

Let's add all the required "u-boot,dm-spl" for SPL config.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>
diff --git a/arch/riscv/dts/ae350_32.dts b/arch/riscv/dts/ae350_32.dts
index 7057684..083f676 100644
--- a/arch/riscv/dts/ae350_32.dts
+++ b/arch/riscv/dts/ae350_32.dts
@@ -3,6 +3,7 @@
 /dts-v1/;
 
 #include "binman.dtsi"
+#include "ae350-u-boot.dtsi"
 
 / {
 	#address-cells = <1>;