Add qemu-riscv@nongnu.org as the RISC-V list

Add qemu-riscv@nongnu.org as the RISC-V list

We now have a RISC-V specific QEMU development list.

Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>