doc: qemu-riscv: Update QEMU run command

Explicitly pass the "-bios" option to QEMU to run U-Boot, instead
of the "-kernel" option, as we know that "-bios" behavior will be
changed since QEMU 5.1.0.

This also updates validated QEMU version to 5.0.0.

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