cpufreq: ARM big LITTLE DT: Return CPUFREQ_ETERNAL if clock-latency isn't found

If "/cpus" node isn't present or "clock-latency" isn't defined we are
returning error currently. Let's return CPUFREQ_ETERNAL instead, so
that we don't fail.

Flag appropriate messages to user in such cases.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>