mfd: 88pm886: Constify struct regmap_irq_chip and some other structures

'struct regmap_irq_chip' 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.

While at it, also constify some other structures.

On a x86_64, with allmodconfig, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
   5032	   3304	     64	   8400	   20d0	drivers/mfd/88pm886.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
   5800	   2536	     64	   8400	   20d0	drivers/mfd/88pm886.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/1681629840761e30494cb8920668710df60a81b8.1746996137.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Lee Jones <lee@kernel.org>