forked from Mirrors/opensbi

Document that when U-Boot v2020.01 (or higher) is used as the payload, we need adjust the instructions a little bit when testing OpenSBI with QEMU 'sifive_u' machine. Signed-off-by: Bin Meng <bmeng.cn@gmail.com> Reviewed-by: Atish Patra <atish.patra@wdc.com>