dt-bindings: cpufreq: remove stale arm_big_little_dt entry

Most of the ARM platforms used v2 OPP bindings to support big-little
configurations. This arm_big_little_dt binding is incomplete and was
never used.

Commit f174e49e (cpufreq: remove unused arm_big_little_dt driver)
removed the driver supporting this binding, but the binding was left
unnoticed, so let's get rid of it now.

Fixes: f174e49e (cpufreq: remove unused arm_big_little_dt driver)
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>