dt-bindings: thermal: rcar-thermal: add R8A77990 support

Document the R-Car E3 (R8A77990) SoC bindings.

Signed-off-by: Yoshihiro Kaneko <ykaneko0929@gmail.com>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Tested-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>