hw/openrisc: Add the OpenRISC virtual machine

hw/openrisc: Add the OpenRISC virtual machine

This patch adds the OpenRISC virtual machine 'virt' for OpenRISC.  This
platform allows for a convenient CI platform for toolchain, software
ports and the OpenRISC linux kernel port.

Much of this has been sourced from the m68k and riscv virt platforms.

The platform provides:
 - OpenRISC SMP with up to 4 cpus
 - A virtio bus with up to 8 devices
 - Standard ns16550a serial
 - Goldfish RTC
 - SiFive TEST device for poweroff and reboot
 - Generated Device Tree to automatically configure the guest kernel

Signed-off-by: Stafford Horne <shorne@gmail.com>