PM / devfreq: 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
    621	    176	      0	    797	    31d	drivers/devfreq/governor_userspace.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   670	    144	      0	    814	    32e	drivers/devfreq/governor_userspace.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>