dt-bindings: thermal: Get rid of thermal.txt and replace references

Now that we have yaml bindings for the thermal subsystem, get rid of the
old bindings (thermal.txt).

Replace all references to thermal.txt in the Documentation with a link
to the appropriate YAML bindings using the following search and replace
pattern:
 - If the reference is specific to the thermal-sensor-cells property,
 replace with a pointer to thermal-sensor.yaml
 - If the reference is to the cooling-cells property, replace with a
 pointer to thermal-cooling-devices.yaml
 - If the reference is generic thermal bindings, replace with a
 reference to thermal*.yaml.

Signed-off-by: Amit Kucheria <amit.kucheria@linaro.org>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/e9aacd33071a00568b67e110fa3bcc4d86d3e1e4.1595245166.git.amit.kucheria@linaro.org