iommu/arm-smmu-v3: Add second level of context descriptor table

The SMMU can support up to 20 bits of SSID. Add a second level of page
tables to accommodate this. Devices that support more than 1024 SSIDs now
have a table of 1024 L1 entries (8kB), pointing to tables of 1024 context
descriptors (64kB), allocated on demand.

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