cpufreq: pmac32: use cpufreq_generic_init()

Use generic cpufreq_generic_init() routine instead of replicating the same code
here.

Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>