gpio: Constify struct configfs_item_operations and configfs_group_operations

gpio: Constify struct configfs_item_operations and configfs_group_operations

'struct configfs_item_operations' and 'configfs_group_operations' are not
modified in these drivers.

Constifying these structures moves some data to a read-only section, so
increases overall security, especially when the structure holds some
function pointers.

On a x86_64, with allmodconfig, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  43935	  11632	    384	  55951	   da8f	drivers/gpio/gpio-aggregator.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  44191	  11376	    384	  55951	   da8f	drivers/gpio/gpio-aggregator.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/ccb5155342ce6dbb89cfbad0687b448860d8e8f0.1765703044.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@oss.qualcomm.com>