powercap: dtpm: Constify some structures

'struct powercap_zone_constraint_ops', 'struct powercap_zone_ops' and the
'dtpm_node_callback_t' array are not modified in this driver.

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

On a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  22049	   7736	    128	  29913	   74d9	drivers/powercap/dtpm.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  22273	   7512	    128	  29913	   74d9	drivers/powercap/dtpm.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Abel Vesa <abel.vesa@oss.qualcomm.com>
Link: https://patch.msgid.link/a4737f7c69e044d35348c1e9c7f69f229070d29b.1793524483.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>