cpufreq: pmac64: enable cpufreq on iMac G5 (iSight) model

Enable cpufreq on iMac G5 (iSight) model. Tested with the 2.1 GHz version.

Signed-off-by: Aaro Koskinen <aaro.koskinen@iki.fi>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>