dt-bindings: thermal: rcar-gen3-thermal: Add r8a774b1 support

Document RZ/G2N (R8A774B1) SoC bindings.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/1569248589-52372-1-git-send-email-biju.das@bp.renesas.com