[POWERPC] powermac: Fix G5-cpufreq for cpu on/offline

The original code here is wrong, it applies "previous" knowledge.
The way the cpufreq core is designed is that the policy for the
secondary CPU that comes online says that it must in fact not
use this policy but use the same as the other CPUs that are
listed, which in fact is CPU#0.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>