iommu: io-pgtable-arm: Add selftest for mixed block + page mappings

There can be page table bugs when both block and page mappings are used
to make up the mapping for a single VA range.  Add a test case for this
to the selftests.

Change-Id: Ic2b943dd74f1ed2ed1e5e03832742f0e6deff58e
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>