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 MT2712.

Signed-off-by: Louis Yu <louis.yu@mediatek.com>
Reviewed-by: Dawei Chien <dawei.chien@mediatek.com>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>