thermal: sti: Remove obsolete platforms from the DT doc.

STiH415/6 SoC's are being removed from the kernel. This
patch removes the compatibles from the dt doc and also
updates the example to a supported platform.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Cc: <rui.zhang@intel.com>
Cc: <edubezval@gmail.com>
Cc: <robh+dt@kernel.org>
Acked-by: Rob Herring <robh@kernel.org>