iommu/arm-smmu: do a single 64-bit write for ATOS

Hardware requires that the ATOS command be issued with a single 64-bit
write instead of two 32-bit writes as we're currently doing.  Fix this.

Change-Id: I43104c89c2f27b75d1176c8cbcd214666321244f
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>