thermal: rcar_gen3_thermal: Add r8a774b1 support

Add r8a774b1 specific compatible string.

Signed-off-by: Biju Das <biju.das@bp.renesas.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/1569248746-56718-1-git-send-email-biju.das@bp.renesas.com