documentation/iommu: Add desc for opt-out-tbu-halting, min-iova-align

Add description for qcom,opt-out-tbu-halting binding which
enables TBUs to opt-out from being halted during ATOS and
qcom,min-iova-align binding which helps align the start and
end of the mapped buffers to prefetch size boundary.

Change-Id: Iefa2d96516c8ca74d0f32f2d6570226f753e4784
Signed-off-by: Swathi Sridhar <swatsrid@codeaurora.org>