iommu/arm-smmu-v3: Set MEV bit in nested STE for DoS mitigations

There is a DoS concern on the shared hardware event queue among devices
passed through to VMs, that too many translation failures that belong to
VMs could overflow the shared hardware event queue if those VMs or their
VMMs don't handle/recover the devices properly.

The MEV bit in the STE allows to configure the SMMU HW to merge similar
event records, though there is no guarantee. Set it in a nested STE for
DoS mitigations.

In the future, we might want to enable the MEV for non-nested cases too
such as domain->type == IOMMU_DOMAIN_UNMANAGED or even IOMMU_DOMAIN_DMA.

Link: https://patch.msgid.link/r/8ed12feef67fc65273d0f5925f401a81f56acebe.1741719725.git.nicolinc@nvidia.com
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Reviewed-by: Pranjal Shrivastava <praan@google.com>
Acked-by: Will Deacon <will@kernel.org>
Signed-off-by: Nicolin Chen <nicolinc@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>

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