cpufreq: arm_big_little: make cpufreq_arm_bL_ops structures const

Make these const as they are only getting passed to the functions
bL_cpufreq_{register/unregister} having the arguments as const.

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