arm: highmem: Add support for flushing kmap_atomic mappings

The highmem code provides kmap_flush_unused to ensure all kmap
mappings are really removed if they are unused. This code does not
handle kmap_atomic mappings since they are managed separately.
This prevents an issue for any code which relies on having absolutely
no mappings for a particular page. Rather than pay the penalty of
having CONFIG_DEBUG_HIGHMEM on all the time, add functionality
to remove the kmap_atomic mappings in a similar way to kmap_flush_unused.

Change-Id: I9d73abad693c18f2daa1647353e7592b255475b0
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>