cpufreq: cpufreq_stats: constify attribute_group structures

attribute_groups are not supposed to change at runtime. All functions
working with attribute_groups provided by <linux/sysfs.h> work with const
attribute_group. So mark the non-const structs as const.

File size before:
   text	   data	    bss	    dec	    hex	filename
   1655	    256	      4	   1915	    77b	drivers/cpufreq/cpufreq_stats.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   1695	    192	      4	   1891	    763	drivers/cpufreq/cpufreq_stats.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>