iommu/arm-smmu: Do a local halt around impl def programming

As per the hardware spec, the SMMU must be halted during programming of
the implementation defined registers.  Add the missing halt.

Change-Id: Id28b546e5b5b646c263d7431a32f3c95ed3f2042
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>