dt-bindings: ap806: add the thermal node in the syscon file

Explain the thermal bindings now that the thermal IP is described being
inside of a system controller. Add a reference to the thermal-zone node.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>