asm-generic: remove ISA_DMA_THRESHOLD in scatterlist.h

This is the first half of the attempt to use asm-generic/scatterlist.h
on every architecture.

There are only two ways to define scatterlist structure. So it's easy
to convert every architecture to use asm-generic/scatterlist.h.

This patch:

The trick for ISA_DMA_THRESHOLD in asm-generic/scatterlist.h doesn't work
for powerpc.  This lets architectures defin ISA_DMA_THRESHOLD.

Hopefully, we can remove ISA_DMA_THRESHOLD in the future; we can do better
to decide if the bouncing is necessary or not.

Signed-off-by: FUJITA Tomonori <fujita.tomonori@lab.ntt.co.jp>
Cc: Arnd Bergmann <arnd@arndb.de>
Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Matt Turner <mattst88@gmail.com>
Cc: Russell King <linux@arm.linux.org.uk>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>