Documentation: devicetree: add binding for Broadcom STB AVS TMON

Add binding for Broadcom STB thermal.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Markus Mayer <mmayer@broadcom.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>