riscv: ae350: Fix OF_BOARD boot failure

Enable OF_HAS_PRIOR_STAGE for ae350 boards with OF_BOARD

Fixes: 239d22c7 ("fdt: Enable OF_HAS_PRIOR_STAGE for most boards with OF_BOARD")
Signed-off-by: Rick Chen <rick@andestech.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>