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

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>