docs/memory-barriers.txt/kokr: Add memory barrier dma_mb()

Translate this commit to Korean:

    ed59dfd9 ("asm-generic: Add memory barrier dma_mb()")

Signed-off-by: SeongJae Park <sj@kernel.org>
Reviewed-by: Yunjae Lee <lyj7694@gmail.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>