iommu/arm-smmu: Collect forward declarations at the top

We've been slowly littering arm-smmu.c with forward declarations all
over the place, moving them around when they're not high enough in the
file, etc.  Just move them all to the top of the file once and for all.

Change-Id: I8f2f075546f287db17f1b6e3b08390296cb9b9ff
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>