iommu: arm-smmu: update access to SMMU TCU testbus registers with scm call

Update access to SMMU TCU testbus and testbus_sel registers using scm.

Change-Id: I5a125070977ffa061a8ee94fdbd9f56603c00a82
Signed-off-by: Faiyaz Mohammed <faiyazm@codeaurora.org>
[guptap@codeaurora.org: resolved trival merge conflicts]
Signed-off-by: Prakash Gupta <guptap@codeaurora.org>