Update the defconfig:
 - Add CONFIG_HW_RANDOM=y and CONFIG_HW_RANDOM_VIRTIO=y to enable
   VirtIORNG when running on QEMU
Signed-off-by: NAlistair Francis <alistair.francis@wdc.com>
Signed-off-by: NPaul Walmsley <paul.walmsley@sifive.com>