The mb() is the superset of dma and smp. Using bar.xxx to implement
mb() will cause problem when sync data with dma device, becasue
bar.xxx couldn't guarantee bus transactions finished at outside bus
level.

We must use sync.s instead of bar.xxx for dma data synchronization
and it will guarantee retirement after getting the bus bresponse.

Changes for V2:
 - Use sync.s for all mb, rmb, wmb, dma_wmb, dma_rmb.
Signed-off-by: Guo Ren <ren_guo@c-sky.com>
Cc: Arnd Bergmann <arnd@arndb.de>