powerpc/aim: Update timebase directly on resume instead of through platform

This only works on single-CPU G4 systems, and more work is needed for
dual-CPU systems. That said, platform sleep does not work, and this is
currently only used for PMU-based CPU speed change.

The elimination of the platform_smp_timebase_sync() call is so that the
timebase sync rendezvous can be enhanced to perform better
synchronization, which requires a full rendezvous. This would be
impossible to do on this single-threaded run.