Add declaration and cacheable_memcpy().  I'll be needing this function in
new 4xx EMAC driver I'm going to submit to netdev soon.

IMHO, the better place for the declaration would be asm-powerpc/string.h,
unfortunately, ppc64 doesn't have this function, so asm-ppc/system.h is the
next best place.
Signed-off-by: Eugene Surovegin <ebs@ebshome.net>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>