iommu/arm-smmu-v3: Update comment about STE liveness

Update the comment to reflect the fact that the STE is not always
installed. arm_smmu_domain_finalise_s1 intentionnaly calls
arm_smmu_write_ctx_desc while the STE is not installed.

Reviewed-by: Nicolin Chen <nicolinc@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Signed-off-by: Michael Shavit <mshavit@google.com>
Tested-by: Nicolin Chen <nicolinc@nvidia.com>
Link: https://lore.kernel.org/r/20230915211705.v8.8.I7a8beb615e2520ad395d96df94b9ab9708ee0d9c@changeid


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