message: [plat] set gPlatResetReason for software reset
  openthread/openthread#5466 allows ot-ctl to reconnect to otbr-agent or
  ot-daemon seamlessly.  However, otbr-agent didn't set gPlatResetReason
  correctly thus this feature is not working for otbr-agent.  This
  commit fix this issue.
diff: @@ -248,6 +248,8 @@ void ControllerOpenThread::Process(const otSysMainloopContext &aMainloop)
  void ControllerOpenThread::Reset(void)
  {
  + gPlatResetReason = OT_PLAT_RESET_REASON_SOFTWARE;
  +
  otInstanceFinalize(mInstance);
  otSysDeinit();
  Init();
  @@ -435,5 +437,6 @@ extern "C" void otPlatLog(otLogLevel aLogLevel, otLogRegion aLogRegion, const ch
  void otPlatReset(otInstance *aInstance)
  {
  OT_UNUSED_VARIABLE(aInstance);
  +
  sReset = true;
  }
  