Add the device tree for the qemu virt machine to the build. The
resulting dtb will be consumed by qemu-system-riscv64.

Co-developed-by: Adrien Ricciardi <aricciardi@baylibre.com>
Signed-off-by: Adrien Ricciardi <aricciardi@baylibre.com>
Signed-off-by: Drew Fustini <dfustini@baylibre.com>