thermal: of-thermal: add API for getting sensor ID from DT

This patch adds new API thermal_zone_of_get_sensor_id() to
provide the feature of getting sensor ID from DT thermal
zone's node. It's useful for thermal driver to register the
specific thermal zone devices from DT in a common way.

Signed-off-by: Anson Huang <Anson.Huang@nxp.com>
Reviewed-by: Dong Aisheng <aisheng.dong@nxp.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/1582330132-13461-2-git-send-email-Anson.Huang@nxp.com