iio: adc: Constify struct iio_map

iio: adc: Constify struct iio_map

'struct iio_map' are not modified in these drivers.

Constifying this structure moves some data to a read-only section, so
increase overall security.

In order to do it, the prototype of iio_map_array_register() and
devm_iio_map_array_register(), and a few structures that hold a
"struct iio_map *" need to be adjusted.

On a x86_64, with allmodconfig, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  21086	    760	      0	  21846	   5556	drivers/iio/adc/axp20x_adc.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  21470	    360	      0	  21830	   5546	drivers/iio/adc/axp20x_adc.o
  33842	   1697	    384	  35923	   8c53	drivers/iio/addac/ad74413r.o

--
Compile tested only

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://patch.msgid.link/5729dc3cc3892ecf0d8ea28c5f7307b34e27493e.1725729801.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>