thermal: rcar_gen3_thermal: Add r8a77961 support

Add support for the Thermal Sensor/Chip Internal Voltage Monitor in the
R-Car M3-W+ (R8A77961) SoC.

According to the R-Car Gen3 Hardware Manual Errata for Revision 2.00 of
Jan 31, 2020, the thermal parameters for R-Car M3-W+ are the same as for
R-Car M3-W.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Reviewed-by: Niklas Söderlund <niklas.soderlund+renesas@ragnatech.se>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20200306105503.24267-3-geert+renesas@glider.be