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.

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>