dt-bindings: thermal: Add yaml bindings for thermal zones

As part of moving the thermal bindings to YAML, split it up into 3
bindings: thermal sensors, cooling devices and thermal zones.

The thermal-zone binding is a software abstraction to capture the
properties of each zone - how often they should be checked, the
temperature thresholds (trips) at which mitigation actions need to be
taken and the level of mitigation needed at those thresholds.
Signed-off-by: Amit Kucheria <amit.kucheria@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Lukasz Luba <lukasz.luba@arm.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/44e5c68bc654ccaf88945f70dc875fa186dd1480.1585748882.git.amit.kucheria@linaro.org