Input: gpio_keys - 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
   5693	    464	      0	   6157	   180d	drivers/input/keyboard/gpio_keys.o

File size After adding 'const':
   text	   data	    bss	    dec	    hex	filename
   5749	    400	      0	   6149	   1805	drivers/input/keyboard/gpio_keys.o

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