'struct meson_secure_pwrc_domain_desc' is not modified in this driver.

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

On a x86_64, with allmodconfig, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
   4909	   4072	      0	   8981	   2315	drivers/pmdomain/amlogic/meson-secure-pwrc.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
   8605	    392	      0	   8997	   2325	drivers/pmdomain/amlogic/meson-secure-pwrc.o
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/871d6b708de8bb42e1fabd8a601dc9a9a217cf00.1719863475.git.christophe.jaillet@wanadoo.frSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>