iommu: arm-smmu: Add struct for tracking tcu specific data

Future changes require more than just a list to be stored for the
implementation-defined private data for the qsmmuv500 iommu implementation.
Prepare for these changes by moving the tbu list into a containing
structure.

Change-Id: I2dcfba3d7a21225546dfd1b764448c4f2f03edde
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>