dt-bindings: thermal: rcar-thermal: document R8A77970 bindings

Document  the R-Car V3M (R8A77970) SoC in the Renesas R-Car gen2 thermal
bindings. The hardware is the same as  in the R-Car D3 (R8A77995) plus an
extra status register.

Signed-off-by: Sergei Shtylyov <sergei.shtylyov@cogentembedded.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>