of: thermal: Introduce sustainable power for a thermal zone

Introduce an optional property called, sustainable-power, which
represents the power (in mW) which the thermal zone can safely
dissipate.

If provided the property is parsed and associated with the thermal
zone via the thermal zone parameters.

Cc: Zhang Rui <[e-mail]>
Cc: Eduardo Valentin <[e-mail]>
Signed-off-by: Punit Agrawal <[e-mail]>
Signed-off-by: Eduardo Valentin <[e-mail]>

Documentation/devicetree/bindings/thermal/thermal.txt

···
 			by means of sensor ID. Additional coefficients are
 			interpreted as constant offset.
 
+- sustainable-power:	An estimate of the sustainable power (in mW) that the
+  Type: unsigned	thermal zone can dissipate at the desired
+  Size: one cell	control temperature.  For reference, the
+			sustainable power of a 4'' phone is typically
+			2000mW, while on a 10'' tablet is around
+			4500mW.
+
 Note: The delay properties are bound to the maximum dT/dt (temperature
 derivative over time) in two situations for a thermal zone:
 (i)  - when passive cooling is activated (polling-delay-passive); and
···
 		 * z = c1*x1 + c2*x2 + c3*x3
 		 */
 		coefficients =		<1200	-345	890>;
+
+		sustainable-power = <2500>;
 
 		trips {
 			/* Trips are based on resulting linear equation */

drivers/thermal/of-thermal.c

···
 	for_each_child_of_node(np, child) {
 		struct thermal_zone_device *zone;
 		struct thermal_zone_params *tzp;
+		u32 prop;
 
 		/* Check whether child is enabled or not */
 		if (!of_device_is_available(child))
···
 
 		/* No hwmon because there might be hwmon drivers registering */
 		tzp->no_hwmon = true;
+
+		if (!of_property_read_u32(child, "sustainable-power", &prop))
+			tzp->sustainable_power = prop;
 
 		zone = thermal_zone_device_register(child->name, tz->ntrips,
 						    0, tz,