docs/platform: Update QEMU parameter for fw_payload

Since QEMU v5.1, if there is no "-bios" option provided, the default
OpenSBI firmware will be loaded by QEMU as the BIOS automatically,
hence to load fw_payload type image, we should explicitly pass the
"-bios" option to QEMU.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Atish Patra <atish.patra@wdc.com>