iommu/arm-smmu-v3: Manage ASIDs with xarray

In preparation for sharing some ASIDs with the CPU, use a global xarray to
store ASIDs and their context. ASID#0 is now reserved, and the ASID
space is global.

Signed-off-by: Jean-Philippe Brucker <jean-philippe@linaro.org>
Link: https://lore.kernel.org/r/20200519175502.2504091-9-jean-philippe@linaro.org


Signed-off-by: Will Deacon <will@kernel.org>