iommu: Add note about struct iommu_fwspec usage

This structure is to be considered private to the IOMMU API. Except for
very few exceptions, IOMMU consumer drivers should treat this as opaque
data.
Acked-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20221206165945.3551774-2-thierry.reding@gmail.comSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>

include/linux/iommu.h

@@ -605,6 +605,10 @@ struct iommu_group *fsl_mc_device_group(struct device *dev);
  * @flags: IOMMU_FWSPEC_* flags
  * @num_ids: number of associated device IDs
  * @ids: IDs which this device may present to the IOMMU
+ *
+ * Note that the IDs (and any other information, really) stored in this structure should be
+ * considered private to the IOMMU device driver and are not to be used directly by IOMMU
+ * consumers.
  */
 struct iommu_fwspec {
 	const struct iommu_ops	*ops;