dt-bindings: thermal: rcar-gen3-thermal: document R8A77980 bindings

Document the R-Car V3H (R8A77980) SoC in the Renesas R-Car gen3 thermal
bindings.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>