thermal/drivers: imx: Fix missing of_node_put() at probe time

After finishing using cpu node got from of_get_cpu_node(), of_node_put()
needs to be called.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/1585232945-23368-1-git-send-email-Anson.Huang@nxp.com