pmdomain: amlogic: Constify some structures

Most structures in this driver are not modified.

Constifying these structures moves some data to a read-only section, so
increase overall security, especially when the structure holds some
function pointers. (This is the case for see meson_ee_pwrc_domain_desc)

On a x86_64, with allmodconfig, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
   8924	   3832	      0	  12756	   31d4	drivers/pmdomain/amlogic/meson-ee-pwrc.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  12396	    336	      0	  12732	   31bc	drivers/pmdomain/amlogic/meson-ee-pwrc.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Link: https://lore.kernel.org/r/edc560afe2a8763c93341d161daeb8b33ba606c6.1746199917.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>