dt-bindings: thermal: Allow multiple devices to share cooling map

Allow cooling devices sharing same trip point with same contribution
value to share the cooling map as well. Otherwise the same information
will be duplicated for each device sharing the trip point.

Signed-off-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>