xtensa: support DMA to high memory

- don't bugcheck if high memory page is passed to xtensa_map_page;
- turn empty dcache flush macros into functions so that they could be
  passed as function parameters;
- use kmap_atomic to map high memory pages for cache invalidation/
  flushing performed by xtensa_sync_single_for_{cpu,device}.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>