iommu/of: return -EPROBE_DEFER when smmu driver is not registered

Return -EPROBE_DEFER from of_iommu_xlate() for the smmu client devices
that are on the platform bus but trying to register their drivers before
SMMU driver registration happens. Without this deferral probe can result
into a situation of not adding the smmu clients to any of the iommu
groups thus can't use iommu_*() services.

Change-Id: I7b9da4ea1bc3bdcfb17e3c81b85938d50544fb9d
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>