dt-bindings: thermal: Add binding document for Mediatek thermal controller

This adds the device tree binding documentation for the mediatek thermal
controller found on Mediatek MT8173 and other SoCs.

Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>
Reviewed-by: Daniel Kurtz <djkurtz@chromium.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Eduardo Valentin <edubezval@gmail.com>