dt-bindings: thermal: add binding documentation for UniPhier thermal monitor

Add devicetree binding documentation for thermal monitor implemented on
Socionext UniPhier SoCs.

Signed-off-by: Kunihiko Hayashi <hayashi.kunihiko@socionext.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Zhang Rui <rui.zhang@intel.com>