documentation/iommu: update description of ARM System MMU binding

This patch adds descriptions for new properties of the device tree
binding for the ARM SMMU architecture. These properties control
arm-smmu driver options.

Change-Id: I480579a5f515780bd8d2989864501a03364ddace
Cc: Grant Likely <grant.likely@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Andreas Herrmann <andreas.herrmann@calxeda.com>
[will: removed device isolation property, as this has been dropped and
       fixed up spacing in documentation]
Signed-off-by: Will Deacon <will.deacon@arm.com>
Git-commit: 0c9d70d72453036195682e64efce77872e9f268b
Git-repo: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git


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