ehci-fsl: fix cache coherency problem on system with large memory

The patch fixes bug http://bugzilla.kernel.org/show_bug.cgi?id=7482.

It sets USB snooping on 4G space for PowerPC platforms without
CONFIG_NOT_COHERENT_CACHE defined.

Reported-by: Stefan Meyer <reyems@telkomsa.net>
Signed-off-by: Li Yang <leoli@freescale.com>
Cc: Greg KH <greg@kroah.com>
Cc: David Brownell <david-b@pacbell.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>