cpufreq: Enable big.LITTLE cpufreq driver on arm64

There are arm64 big.LITTLE systems so enable the big.LITTLE cpufreq driver.
While IKS is not available for these systems the driver is still useful
since it manages clusters with shared frequencies which is the common case
for these systems.

Long term combining the cpufreq-cpu0 and big.LITTLE drivers may be a
more sensible option but that is substantially more complex especially
in the case of IKS.

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