documentation/iommu: Remove description for qcom,min-iova-align

Remove description for qcom,min-iova-align since the support
to align the start and end of all mapped buffers to
ARM_SMMU_MIN_IOVA_ALIGN has been removed.

Change-Id: I15f4f0338962276396e7e916f8f003d879c82c4b
Signed-off-by: Swathi Sridhar <swatsrid@codeaurora.org>