iommu: io-pgtable-arm: unmap last level all at once

Currently we walk each last-level leaf pte during unmap and zero them
out individually.  Since these last-level ptes are all contiguous (up to
512 entries), optimize the unmapping process by simply zeroing them all
out at once rather than operating on them individually.

Change-Id: I21d490e8a94355df4d4caecab33774b5f8ecf3ca
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Patrick Daly <pdaly@codeaurora.org>