Add CVA6 (the corev_apu from the fpga) model from https://github.com/openhwgroup/cva6
Previous submission: https://lists.gnu.org/archive/html/qemu-devel/2025-06/msg01411.html v4: - comment fixes - make the core-type first and then add machine v3: - fix missing file source - set 64bit only for now v2: - rebased and fixed whitespace issues
