thermal/drivers/thermal_mmio: Drop of_match_ptr()

Now that the driver depends on OF, we know what of_match_ptr() will
always resolve to, so we might as well save cpp some work.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>