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.com


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>