cpufreq: arm_big_little: Make ->get_transition_latency() mandatory

All users of arm_big_little driver are defining it and there is no need
to keep it optional.

Make it mandatory to remove the always true conditional statement.

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