Cross Memory Attach: make it Kconfigurable

Add a Kconfig option to allow people who don't want cross memory attach to
not have it included in their build.

Signed-off-by: Chris Yeoh <yeohc@au1.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>