Input: ims-pcu - 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
  13547	   1600	      0	  15147	   3b2b	drivers/input/misc/ims-pcu.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
  13675	   1472	      0	  15147	   3b2b	drivers/input/misc/ims-pcu.o

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Dmitry Torokhov <dmitry.torokhov@gmail.com>