thermal: Constify struct thermal_zone_device_ops

'struct thermal_zone_device_ops' are not modified in these drivers.

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, as an example:
Before:
======
   text	   data	    bss	    dec	    hex	filename
  28116	   5168	    128	  33412	   8284	drivers/thermal/armada_thermal.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
  28244	   5040	    128	  33412	   8284	drivers/thermal/armada_thermal.o

Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Frank Li <Frank.Li@nxp.com>
Reviewed-by: Miquel Raynal <miquel.raynal@bootlin.com> # For Armada
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Link: https://lore.kernel.org/r/5bba3bf0139e2418b306a0f9a2f1f81ef49e88a6.1748165978.git.christophe.jaillet@wanadoo.fr


Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>