iommu/arm-smmu: Remove support for reg_[read/write] functions

Remove the reg_[read/write] functions from the SMMU driver
and the IOMMU core, as they are not being used by anyone.

Change-Id: Ifde9cef2bf731bef2f6a2b0412a10b26cbe779b4
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>