device_get_prop
error = device_get_prop(dev, DEV_PROP_NAME_IOMMU,
int device_get_prop(device_t dev, const char *name, void **valp);
error = device_get_prop(dev, device_get_nameunit(unit->iommu.dev),
error = device_get_prop(dev, DEV_PROP_NAME_IOMMU, (void **)&iommu_dev);