mfd: ab8500-core: 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
  16298	   1009	    184	  17491	   4453	drivers/mfd/ab8500-core.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
  16490	    817	    184	  17491	   4453	drivers/mfd/ab8500-core.o
Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>

drivers/mfd/ab8500-core.c

@@ -1059,15 +1059,15 @@ static struct attribute *ab9540_sysfs_entries[] = {
 	NULL,
 };
 
-static struct attribute_group ab8500_attr_group = {
+static const struct attribute_group ab8500_attr_group = {
 	.attrs	= ab8500_sysfs_entries,
 };
 
-static struct attribute_group ab8505_attr_group = {
+static const struct attribute_group ab8505_attr_group = {
 	.attrs	= ab8505_sysfs_entries,
 };
 
-static struct attribute_group ab9540_attr_group = {
+static const struct attribute_group ab9540_attr_group = {
 	.attrs	= ab9540_sysfs_entries,
 };