dt-bindings: thermal: Add binding document for SR thermal

Add binding document for supported thermal implementation
in Stingray.

Reviewed-by: Ray Jui <ray.jui@broadcom.com>
Reviewed-by: Scott Branden <scott.branden@broadcom.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Pramod Kumar <pramod.kumar@broadcom.com>
Signed-off-by: Srinath Mannam <srinath.mannam@broadcom.com>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>