iommu: Introduce accessors for iommu private data

Add dev_iommu_priv_get/set() functions to access per-device iommu
private data. This makes it easier to move the pointer to a different
location.

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/20200326150841.10083-9-joro@8bytes.org