iommu/arm-smmu: Remove unused functions

The iommu_reg_{read/write} routines are no longer used
by any drivers. The usage of these functions were removed
previously due to security concerns. Hence, chop out the
code as they are left unused.

Change-Id: I171426706f6aa9958ab93787dda411b087760328
Signed-off-by: Sudarshan Rajagopalan <sudaraja@codeaurora.org>