arm: dma-mapping: flush highmem mappings

DMA allocations with no kernel mapping are likely
protected as well. Stale highmem mappings in such cases
with cache-able attributes may lead to speculative fetch,
as highmem mappings are not cleared out.

Hence flush out unused highmem mappings explicitly when
allocation request with no kernel mapping is requested.

Change-Id: Ic1de633c6364eaa1b6d5b0932f2cfe17d64d920e
Signed-off-by: Shiraz Hashim <shashim@codeaurora.org>