iommu/arm-smmu: remove support for chained SMMUs

The ARM SMMU driver has supported chained SMMUs (i.e. SMMUs connected
back-to-back in series) via the smmu-parent property in device tree.
This was in anticipation of somebody building such a configuration,
however that seems not to be the case.

This patch removes the unused chained SMMU hack from the driver. We can
consider adding it back later if somebody decided they need it, but for
the time being it's just pointless mess that we're carrying in mainline.

Removal of the feature also makes migration to the generic IOMMU bindings
easier.

Signed-off-by: Will Deacon <will.deacon@arm.com>