dt-bindings: Document the hi3660 thermal sensor binding

This adds documentation of device tree bindings for the
thermal sensor controller of hi3660 SoC.

Signed-off-by: Kevin Wangtao <kevin.wangtao@linaro.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wei Xu <xuwei5@hisilicon.com>