iommu/io-pgtable-arm: sync map update before page table walk

'commit 87a91b15 ("iommu/io-pgtable-arm: Centralise sync points")'
added barrier at the end of map so in the case of a coherent IOMMU, to
ensure the page table updates are fully synchronized against a subsequent
page table walk. map_sg would also need the same support.

Change-Id: I3dc3e5cdc133f3b5fc312784fdc98a287e3c83fd
Signed-off-by: Prakash Gupta <guptap@codeaurora.org>