powerpc/4xx: Fix typos in kexec config dependencies

Kexec is not supported on 47x. 47x is a variant of 44x with slightly
different MMU and SMP support. There was a typo in the config dependency
for kexec. This patch fixes the same.

Signed-off-by: Suzuki K. Poulose <suzuki@in.ibm.com>
Signed-off-by: Paul Bolle <pebolle@tiscali.nl>
Cc:	Kumar Gala <galak@kernel.crashing.org>
Cc:	Josh Boyer <jwboyer@gmail.com>
Cc:	linux ppc dev <linuxppc-dev@lists.ozlabs.org>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>