iommu: io-pgtable-fast: Fix IOMMU configuration usage in self-tests

When the fastmap selftests are running, a half-baked configuration
structure is allocated on the stack for the sole purpose of exercising
the mapping and unmapping paths. This leads to issues, as the
implementation of av8l_clean_range() assumes that the configuration
structure is well-defined (i.e. the device pointer is meaningful,
and can be used for applying cache-maintenance, and is contained in an
iopgtable structure). To avoid this, change av8l_clean_range() so that
it no longer assumes that the configuration structure is contained in an
iopgtable structure, as well as update the coherency settings used for
the selftests, so that cache-maintenance is not performed. It is
pointless to perform cache-mainteance for the tests, as no device
access occurs.

Change-Id: I428b893132e57184eb9f97d656ed2357489bb230
Signed-off-by: Isaac J. Manjarres <isaacm@codeaurora.org>