doc: sifive: fu540: Update QEMU instruction when using U-Boot as the payload

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>