riscv: spl: andes: Move the DTB in front of kernel

Originally, u-boot SPL will place the DTB directly after the kernel,
but the size of the kernel does not include the BSS section, This
means that u-boot SPL places the DTB in the kernel BSS section causing
the DTB to be cleared by the kernel BSS initialisation.

Moving the DTB in front of the kernel can avoid this error.

Signed-off-by: Randolph <randolph@andestech.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>