Documentation/DMA-API-HOWTO: add ARCH_KMALLOC_MINALIGN description

Add ARCH_KMALLOC_MINALIGN description in "Platform Issues" section.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Matt Mackall <mpm@selenic.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>