iommu/arm-smmu: zero out ptes completely on unmap

Thulium v1 has a hardware bug that requires us to zero out page table
entries completely rather than just setting the valid bits to 0.  The
previous workaround for this [165066b85d: "iommu/arm-smmu: work around
transaction hardware bug"] was actually not quite right since it still
allowed the page frame number to be set in the page table entry.  Fix
this by actually zero'ing out the pte.

Change-Id: I37537874ea4c5d1c00db7de65f70edd93ce63b90
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>