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 <rui.zhang@intel.com>
Cc: Eduardo Valentin <edubezval@gmail.com>
Signed-off-by: Punit Agrawal <punit.agrawal@arm.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>
(cherry picked from commit 647f99255d604aa98f919b89c74567bb4e8fe52c)
Signed-off-by: Kevin Hilman <khilman@linaro.org>