Add the CVA6 (the corev_apu from the fpga) model from https://github.com/openhwgroup/cva6
Tree at: https://gitlab.com/CodethinkLabs/qemu/-/tree/bjdooks/cva6-send-8jun2025?ref_type=heads Fixes: v3: - fix missing file source - set 64bit only for now v2: - rebased and fixed whitespace issues
