iommu: read tcu clock testbus properly

Porgramming guide for tcu clock testbus controller says:

1) Write to the ARM_SMMU_TESTBUS_SEL_HLOS1_NS.
2) Read from the ARM_SMMU_TCU_TESTBUS_HLOS1_NS

to know the internal smmu clock status. Program them accordingly.

Change-Id: I6140d877c2d8366030699fb04099078ebb3b6e2c
Fixes: 47b2cb7a ("iommu: arm-smmu: update access to SMMU TCU testbus registers with scm call")
Signed-off-by: Charan Teja Reddy <charante@codeaurora.org>