'struct regulator_desc' is not modified in this driver.

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

On a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
   3516	   5320	     32	   8868	   22a4	drivers/regulator/mp5416.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
   8508	    320	     32	   8860	   229c	drivers/regulator/mp5416.o

--
Compile tested only
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://patch.msgid.link/fd8d1307f211ec7754f46b6678c04309556003bc.1725807320.git.christophe.jaillet@wanadoo.frSigned-off-by: Mark Brown <broonie@kernel.org>