thermal: add new get_crit_temp callback

Add a new callback so that the generic thermal can get
the critical trip point info of a thermal zone,
which is needed for building the tempX_crit hwmon sysfs attribute.

Signed-off-by: Zhang Rui <rui.zhang@intel.com>
Acked-by: Jean Delvare <khali@linux-fr.org>
Signed-off-by: Len Brown <len.brown@intel.com>