PM / sleep: 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
   3802	    624	     32	   4458	   116a	kernel/power/main.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   3866	    560	     32	   4458	   116a	kernel/power/main.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

kernel/power/main.c

@@ -705,7 +705,7 @@ static struct attribute * g[] = {
 	NULL,
 };
 
-static struct attribute_group attr_group = {
+static const struct attribute_group attr_group = {
 	.attrs = g,
 };