devicetree: Add generic IOMMU device tree bindings

This commit introduces a generic device tree binding for
IOMMU devices.  Only a very minimal subset is described
here, but it is enough to cover the requirements of both the
Exynos System MMU and Tegra SMMU as discussed here:

    https://lkml.org/lkml/2014/4/27/346

Acked-by: Will Deacon <will.deacon@arm.com>
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Reviewed-by: Mark Rutland <mark.rutland@arm.com>
Reviewed-by: Olof Johansson <olof@lixom.net>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>