hw/openrisc/openrisc_sim: Add automatic device tree generation

hw/openrisc/openrisc_sim: Add automatic device tree generation

Using the device tree means that qemu can now directly tell
the kernel what hardware is configured rather than use having
to maintain and update a separate device tree file.

This patch adds automatic device tree generation support for the
OpenRISC simulator.  A device tree is built up based on the state of the
configure openrisc simulator.

This is then dumped to memory and the load address is passed to the
kernel in register r3.

Signed-off-by: Stafford Horne <shorne@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>