iommu/arm-smmu: Ignore some debug registers for QSMMUV2

Debug registers which used to identify TBU/TCU invalidation or SYNC
pending state are only applicable for smmuv500, hence make this change
accordingly.

Change-Id: Iaf64a1fad3bf50324ffea9d91e1d6498546a8cbf
Signed-off-by: Zhenhua Huang <zhenhuah@codeaurora.org>