dt-bindings: thermal: al-thermal: Add binding documentation

Add thermal binding documentation for Amazon's Annapurna Labs Thermal
Sensor.

Signed-off-by: Talel Shenhar <talel@amazon.com>
Reviewed-by: David Woodhouse <dwmw@amazon.co.uk>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>