docs/system: riscv: Add documentation for virt machine

docs/system: riscv: Add documentation for virt machine

This adds detailed documentation for RISC-V `virt` machine,
including the following information:

  - Supported devices
  - Hardware configuration information
  - Boot options
  - Running Linux kernel
  - Running U-Boot

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210627142816.19789-2-bmeng.cn@gmail.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>