mm: highmem: Add definitions for kmap_atomic_flush_unused

kmap_flush_unused does not flush kmap_atomic mappings which
are handled separately. Architectures may have use cases to require
these to be flushed. Add an option to let architectures define
kmap_atomic flushing to get rid of extra mappings.

Change-Id: I5a338ed9e215f0f0ad3ab58a3066d2f4c8ce3ba7
Signed-off-by: Laura Abbott <lauraa@codeaurora.org>