thermal: rcar: Document SoC-specific bindings

The documentation only mentioned the generic fallback compatible property.
Add the missing SoC-specific compatible properties, some of which are
already in use.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Zhang Rui <rui.zhang@intel.com>
Cc: Eduardo Valentin <eduardo.valentin@ti.com>
Cc: linux-pm@vger.kernel.org
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>