iommu: io-pgtable-arm: Correctly unmap the last level

Currently we have an optimization in place for unmapping the last level
of the page tables.  We do this by memset()'ing the entire last level at
once rather than calling unmap on each individual page mapping at the
last level.  For this to work we have to pass in sizes that aren't equal
to any of the supported IOMMU page sizes.  However, our optimization
only applies at the last level.  Unmapping at the other levels still
relies on the fact that unmap is only called with supported IOMMU page
sizes, which means it's currently broken.

Fix this by always calling unmap with an IOMMU page size, unless we're
at the last level of the page tables (i.e. the size to be unmapped is
less than the block size at the second-to-last level), in which case we
can pass in the entire remaining size.

Change-Id: Ie3716002c793af3dca51e0e3363d261f345e9e25
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>