iommu: io-pgtable-arm: make selftests more thorough

The io-pgtable-arm unit tests currently only check a few mappings within
a range to determine whether or not that range "looks good".  There can
be subtle bugs that don't show up until you get to a certain offset
within the range, etc.  Check the entire range before assuming that it's
good.

Change-Id: I244a2150d38f57d95a5c81854cdeaf59ab4ace06
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>