thermal/drivers/thermal_mmio: Constify static struct thermal_mmio_ops

The only usage of thermal_mmio_ops is to pass its address to
devm_thermal_zone_of_sensor_register(), which has a pointer to const
struct thermal_zone_of_device_ops as argument. Make it const to allow
the compiler to put it in read-only memory.

Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Acked-by: Talel Shenhar <talel@amazon.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20210920203849.32136-1-rikard.falkeborn@gmail.com