iommu/arm-smmu: add support for dynamic stream IDs to DT parsing

Currently we program all stream IDs based on the configuration specified
by clients in their device tree nodes, so we've always required any
`iommus' entries to have *exactly* 1 stream ID per list element.
However, there are some devices that might not have statically-defined
SIDs that can be placed in the device tree (devices behind PCI-e, for
example).  Add support for this by accepting stream-ID-less `iommus'
entries in the device tree.

Change-Id: I62745dc4159114181fa51b22c732935cb47f60bc
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>