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 <rui.zhang@intel.com>
Cc: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: NPunit Agrawal <punit.agrawal@arm.com>
Signed-off-by: NEduardo Valentin <edubezval@gmail.com>