dt-bindings: thermal: make cooling-maps property optional

Cooling-maps doesn't have to be a required property because there may
be no cooling device on system, or there are no enough cooling devices for
each thermal zone in multiple thermal zone cases since cooling devices
can't be shared.
So make this property optional to remove such limitations.

For thermal zones with no cooling-maps, there could be critic trips
that can trigger CPU reset or shutdown. So they still can take actions.
Signed-off-by: Yuantian Tang <andy.tang@nxp.com>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200309045411.21859-1-andy.tang@nxp.com

Documentation/devicetree/bindings/thermal/thermal.txt

@@ -142,11 +142,11 @@ Required properties:
 - trips:		A sub-node which is a container of only trip point nodes
   Type: sub-node	required to describe the thermal zone.
 
+Optional property:
 - cooling-maps:		A sub-node which is a container of only cooling device
   Type: sub-node	map nodes, used to describe the relation between trips
 			and cooling devices.
 
-Optional property:
 - coefficients:		An array of integers (one signed cell) containing
   Type: array		coefficients to compose a linear relation between
   Elem size: one cell	the sensors listed in the thermal-sensors property.