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

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

The property #thermal-sensor-cells is required in each device that acts
as a thermal sensor. It is used to uniquely identify the instance of the
thermal sensor inside the system.

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/a91b5603caea5b8854cc9f5325448e4c7228c328.1585748882.git.amit.kucheria@linaro.org