iommu/arm-smmu-v3: Prepare arm_smmu_s1_cfg for SSID support

When adding SSID support to the SMMUv3 driver, we'll need to manipulate
leaf pasid tables and context descriptors. Extract the context
descriptor structure and align with the way stream tables are handled.

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Signed-off-by: Will Deacon <will@kernel.org>