iommu/arm-smmu: Implement .reg_read and .reg_write

Implement the register reader and writer functions so that clients can
peek and poke their SMMU's registers.

Change-Id: I9dac59a16b42fec6b2a7639603289911f88b067d
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>