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>

Documentation/devicetree/bindings/iommu/arm,smmu.txt

@@ -48,6 +48,12 @@ conditions.
                   from the mmu-masters towards memory) node for this
                   SMMU.
 
+- calxeda,smmu-secure-config-access : Enable proper handling of buggy
+                  implementations that always use secure access to
+                  SMMU configuration registers. In this case non-secure
+                  aliases of secure registers have to be used during
+                  SMMU configuration.
+
 Example:
 
         smmu {