docs/system: Add RISC-V documentation

docs/system: Add RISC-V documentation

Add RISC-V system emulator documentation for generic information.
`Board-specific documentation` and `RISC-V CPU features` are only
a placeholder and will be added in the future.

Signed-off-by: Bin Meng <bin.meng@windriver.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Message-id: 20210126060007.12904-9-bmeng.cn@gmail.com
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>