thermal: rcar_gen3_thermal: Document the R-Car Gen3

Signed-off-by: Hien Dang <hien.dang.eb@renesas.com>
Signed-off-by: Khiem Nguyen <khiem.nguyen.xt@renesas.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>