DTU: ignore writes on readonly DtuRegs.
DTU: ignore writes on readonly DtuRegs.

Instead of assert(false), as we did before.

TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu,TUD-OS/gem5-dtu