[POWERPC] Rework SMP timebase handoff for pasemi

Rework timebase handoff to play nice with configurations with more than
2 cores, as well as with CPU hotplug.

Previous scheme just pushed out the current timebase from the giving
core to all cores without caring if they wanted it or not, nor checking
if they'd taken it. The taking side didn't make sure the giving side
had provided a value yet either. In other words, it was completely broken.

Signed-off-by: Olof Johansson <olof@lixom.net>
Signed-off-by: Paul Mackerras <paulus@samba.org>