[POWERPC] PS3: Fix CONFIG_SMP=n, CONFIG_KEXEC=y build

Currently, the ps3 kernel fails to build without smp but with kexec, as
ps3_kexec_cpu_down needs ps3_smp_cleanup_cpu, which isn't defined on UP
kernels. This change adds an empty ps3_smp_cleanup_cpu for UP kernels.

Booted on ps3.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>
Signed-off-by: Geoff Levand <geoffrey.levand@am.sony.com>
Signed-off-by: Paul Mackerras <paulus@samba.org>