iommu/tegra: Add tegra_dev_iommu_get_stream_id() helper

iommu/tegra: Add tegra_dev_iommu_get_stream_id() helper

Access to the internals of struct iommu_fwspec by non-IOMMU drivers is
discouraged. Many drivers for Tegra SoCs, however, need access to their
IOMMU stream IDs so that they can be programmed into various hardware
registers.

Formalize this access into a common helper to make it easier to audit
and maintain.

Acked-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://lore.kernel.org/r/20221206165945.3551774-3-thierry.reding@gmail.com
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>