cpufreq: arm_big_little: make function arguments and structure pointer const

Make the arguments of functions bL_cpufreq_{register/unregister} as
const as the ops pointer does not modify the fields of the
cpufreq_arm_bL_ops structure it points to. The pointer arm_bL_ops is
also getting initialized with ops but the pointer does not modify the
fields. So, make the function argument and the structure pointer const.
Add const to function prototypes too.

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>