cpufreq: arm_big_little_dt: Instantiate as platform_driver

As multiplatform build is being adopted by more and more ARM platforms, initcall
function should be used very carefully. For example, when both arm_big_little_dt
and cpufreq-cpu0 drivers are compiled in, arm_big_little_dt driver may try to
register even if we had platform device for cpufreq-cpu0 registered.

To eliminate this undesired the effect, the patch changes arm_big_little_dt
driver to have it instantiated as a platform_driver. Then it will only run on
platforms that create the platform_device "arm-bL-cpufreq-dt".

Reported-and-tested-by: Rob Herring <robherring2@gmail.com>
Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>