iommu/io-pgtable-arm: Optimize map by batching flushes

Currently, the page table is flushed after the installation of each
individual page table entry.  This is not terribly efficient since
virtual address ranges are often mapped with physically contiguous
chunks of page table memory.  Optimize the map operation by factoring
out the page table flushing so that contiguous ranges of page table
memory can be flushed in one go.

Change-Id: Ie80eb57ef50d253db6489a6f75824d4c746314c7
Signed-off-by: Stepan Moskovchenko <stepanm@codeaurora.org>
Signed-off-by: Mitchel Humpherys <mitchelh@codeaurora.org>
Signed-off-by: Neeti Desai <neetid@codeaurora.org>