mfd: rohm-bd71828: Constify some structures

mfd: rohm-bd71828: Constify some structures

Several structures are not modified in this driver. Constifying them moves
some data to a read-only section, so increases overall security, especially
when the structure holds some function pointers. This is the case for
'gpio_keys_platform_data' and 'mfd_cell'.

On a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  18321	  13952	    192	  32465	   7ed1	drivers/mfd/rohm-bd71828.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  22897	   9376	    192	  32465	   7ed1	drivers/mfd/rohm-bd71828.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Matti Vaittinen <mazziesaccount@gmail.com>
Link: https://lore.kernel.org/r/037e28e587ae899da9acdb45c606d75ec61f858b.1750501700.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Lee Jones <lee@kernel.org>