iommu/io-pgtable-arm: Stricter double-map checking

Currently there's a check to make sure that a mapping isn't being
installed on top of an existing leaf mapping.  However, mapping on top
of an existing table mapping is silently ignored.  Make the check more
strict by testing for any valid page table entry (table, block, or page
descriptor) and by making any overlap fatal.

Change-Id: Ibdffbfdb2b44cf9da5044c2c1828a32979a39760
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>