riscv: ae350: Switch to use binman to generate u-boot.itb

Use the new BINMAN_STANDALONE_FDT option for AE350 based SPL defconfigs,
so that binman is now used to generate u-boot.itb.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Rick Chen <rick@andestech.com>