cpufreq: remove unused arm_big_little_dt driver

Most of the ARM platforms used cpufreq-dt driver irrespective of
whether it's big-little(HMP) or SMP system. This arm_big_little_dt is
not used actively at all.

So let's remove the driver, so that it need not be maintained.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>