dt-bindings: thermal: rcar-thermal: add R8A774C0 support

Document RZ/G2E SoC (a.k.a. r8a774c0) bindings.

Signed-off-by: Fabrizio Castro <fabrizio.castro@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>