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>

drivers/devfreq/governor_userspace.c

@@ -86,7 +86,7 @@ static struct attribute *dev_entries[] = {
 	&dev_attr_set_freq.attr,
 	NULL,
 };
-static struct attribute_group dev_attr_group = {
+static const struct attribute_group dev_attr_group = {
 	.name	= "userspace",
 	.attrs	= dev_entries,
 };