ppc: Minor smp changes for consistency with ppc64

This makes platform code use the smp_ops variable directly instead
of ppc_md.smp_ops, removes the two unused `data' and `wait' arguments
from the *_message_pass() functions, and removes the call to the
never-implemented smp_ops->space_timers() function.

Signed-off-by: Paul Mackerras <paulus@samba.org>