iommu/exynos: Don't unconditionally steal bus ops

Removing the early device registration hook overlooked the fact that
it only ran conditionally on a compatible device being present in the
DT. With exynos_iommu_init() now running as an unconditional initcall,
problems arise on non-Exynos systems when other IOMMU drivers find
themselves unable to install their ops on the platform bus, or at worst
the Exynos ops get called with someone else's domain and all hell breaks
loose.

The global ops/cache setup could probably all now be triggered from the
first IOMMU probe, as with dma_dev assigment, but for the time being the
simplest fix is to resurrect the logic from commit a7b67cd5
("iommu/exynos: Play nice in multi-platform builds") to explicitly check
the DT for the presence of an Exynos IOMMU before trying anything.

Fixes: 928055a0 ("iommu/exynos: Remove custom platform device registration code")
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>