iommu: Move fwspec->iommu_priv to struct dev_iommu

Move the pointer for iommu private data from struct iommu_fwspec to
struct dev_iommu.

Signed-off-by: Joerg Roedel <jroedel@suse.de>
Tested-by: Will Deacon <will@kernel.org> # arm-smmu
Reviewed-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Link: https://lore.kernel.org/r/20200326150841.10083-17-joro@8bytes.org