dt-bindings: thermal: r9a08g045-tsu: Document the TSU unit

dt-bindings: thermal: r9a08g045-tsu: Document the TSU unit

The Renesas RZ/G3S SoC includes a Thermal Sensor Unit (TSU) block designed
to measure the junction temperature. The temperature is measured using
the RZ/G3S ADC, with a dedicated ADC channel directly connected to the TSU.
Add documentation for it.

Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Claudiu Beznea <claudiu.beznea.uj@bp.renesas.com>
Tested-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Link: https://lore.kernel.org/r/20250810122125.792966-2-claudiu.beznea.uj@bp.renesas.com
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>