thermal: rcar_gen3_thermal: Add R-Car Gen3 thermal driver

Add support for R-Car Gen3 thermal sensors. Polling only for now,
interrupts will be added incrementally. Same goes for reading fuses.
This is documented already, but no hardware available for now.

Signed-off-by: Hien Dang <hien.dang.eb@renesas.com>
Signed-off-by: Thao Nguyen <thao.nguyen.yb@rvc.renesas.com>
Signed-off-by: Khiem Nguyen <khiem.nguyen.xt@renesas.com>
Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
[Niklas: document and rework temperature calculation]
Signed-off-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>