hw/riscv: Load OpenSBI as the default firmware

hw/riscv: Load OpenSBI as the default firmware

If the user hasn't specified a firmware to load (with -bios) or
specified no bios (with -bios none) then load OpenSBI by default. This
allows users to boot a RISC-V kernel with just -kernel.

Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Tested-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>