drm/etnaviv: constify static struct cooling_ops

The only usage of cooling_ops is to pass its address to
thermal_of_cooling_device_register(), which takes a pointer to const
struct thermal_cooling_device_ops as input. Make it const to allow the
compiler to put it in read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>