dt-bindings: thermal: sprd: Add the Spreadtrum thermal documentation

Add the Spreadtrum thermal documentation.

Signed-off-by: Baolin Wang <baolin.wang@unisoc.com>
Signed-off-by: Baolin Wang <baolin.wang7@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/444e45ce0a9b390b2502dfcefd1ddb36948fa8e1.1582013101.git.baolin.wang7@gmail.com