power: supply: 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
   2673	    400	      0	   3073	    c01	power/supply/pcf50633-charger.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   2737	    336	      0	   3073	    bed	power/supply/pcf50633-charger.o
Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>

drivers/power/supply/pcf50633-charger.c

@@ -254,7 +254,7 @@ static struct attribute *pcf50633_mbc_sysfs_entries[] = {
 	NULL,
 };
 
-static struct attribute_group mbc_attr_group = {
+static const struct attribute_group mbc_attr_group = {
 	.name	= NULL,			/* put in device directory */
 	.attrs	= pcf50633_mbc_sysfs_entries,
 };