iommu: Add iommu_reg_read and iommu_reg_write

It might be useful for IOMMU clients to peek and poke at their IOMMU's
registers, but knowing how to access those registers is really the job
of the IOMMU driver (it might need to enable specific clocks and
regulators, for example).  Provide an API to read and write IOMMU
registers that can be implemented by the driver.

Change-Id: I5b2f19225f8bd258278780ff24b4ea96460857aa
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>