hw/riscv: Add support for loading a firmware

hw/riscv: Add support for loading a firmware

Add support for loading a firmware file for the virt machine and the
SiFive U. This can be run with the following command:

    qemu-system-riscv64 -machine virt -bios fw_jump.bin -kernel vmlinux

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>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>