Docs: dt: document ARM SMMU generic binding usage

Document how the generic "iommus" binding should be used to describe ARM
SMMU stream IDs instead of the old "mmu-masters" binding.

Change-Id: I6cc28bacedd5af5b3398893e341f5b8cd5733054
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Git-commit: d0acbb75
Git-repo: https://source.codeaurora.org/quic/la/kernel/msm-4.4


Signed-off-by: Patrick Daly <pdaly@codeaurora.org>