iommu/arm-smmu-v3: Write level-1 descriptors atomically

Use WRITE_ONCE() to make sure that the SMMU doesn't read incomplete
stream table descriptors. Refer to the comment about 64-bit accesses,
and add the comment to the equivalent context descriptor code.

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