documentation/iommu: Add description of ARM System MMU binding

This patch adds a description of the device tree binding for the ARM
System MMU architecture.

Cc: Rob Herring <robherring2@gmail.com>
Cc: Andreas Herrmann <andreas.herrmann@calxeda.com>
Cc: Joerg Roedel <joro@8bytes.org>
Acked-by: Grant Likely <grant.likely@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>
Acked-by: Andreas Herrmann <andreas.herrmann@calxeda.com>
Signed-off-by: Joerg Roedel <joro@8bytes.org>