dt-bindings: thermal: rcar-gen3-thermal: Convert bindings to json-schema

Convert Renesas R-Car Gen3 Thermal bindings documentation to
json-schema.

Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Rob Herring <robh@kernel.org>