iommu/arm-smmu-v3: Document ordering guarantees of command insertion

It turns out that we've always relied on some subtle ordering guarantees
when inserting commands into the SMMUv3 command queue. With the recent
changes to elide locking when possible, these guarantees become more
subtle and even more important.

Add a comment documented the barrier semantics of command insertion so
that we don't have to derive the behaviour from scratch each time it
comes up on the list.

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