iommu: arm-smmu: Fix (void *) typecast error

In qsmmuv500_tbu_register(), archdata was being converted into a list
instead of the proper structure. Add a macro in an attempt to prevent
future misuse.

Change-Id: I0703fc341bf55e2174188c5dbb91c314c498f3f9
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>