iommu/iommu-debug: Add dummy driver for standalone testing

The IOMMU test framework relies on the `iommus' property, and we
currently rely on these methods for making that happen:

  (1) Clients enabling their DT nodes.

  (2) We put an `iommus' property in our IOMMU DT nodes themselves.

The problem with (1) is that clients aren't always ready during early
chip validation.  The problem with (2) is that it results in us
recursively mapping into the SMMU when we try to do cache maintenance on
our page table memory.

Fix these problems by introducing a dummy driver with associated device
tree bindings that will do absolutely nothing other than wait for the
SMMU driver and IOMMU test framework to slurp it up.

CRs-Fixed: 1003233
Change-Id: I6a5802aff5bab99d29c6ed9d953a203cbd8015bb
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>