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>