Documentation: update scatterlist struct description in DMA-API-HOWTO

Now we have <asm-generic/scatterlist.h>.  Architectures should use it
instead of inventing the own scatterlist struct.  Let's update the
description.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>