doc: qemu-riscv: Update documentation for QEMU spike machine

We can now use same U-Boot images on both QEMU virt machine and QEMU
spike machine so let's update the QEMU RISC-V documentation.

Signed-off-by: Anup Patel <apatel@ventanamicro.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>