[MTD] 4xx: Don't build arch/ppc dependent drivers in arch/powerpc

These drivers are specific to 4xx support in arch/ppc at the moment.  Make
sure they don't get built on arch/powerpc.

Signed-off-by: Josh Boyer <jwboyer@linux.vnet.ibm.com>
Signed-off-by: Kumar Gala <galak@kernel.crashing.org>