arm64: dts: renesas: r8a77961: Add thermal nodes

Add a device node for the Thermal Sensor/Chip Internal Voltage Monitor
in the R-Car M3-W+ (R8A77961) SoC, and describe the thermal zones.

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>
Link: https://lore.kernel.org/r/20200306110025.24747-1-geert+renesas@glider.be