dt-bindings: thermal: Add YAML schema for sun8i-thermal driver bindings

sun8i-thermal driver supports thermal sensor in wide range of Allwinner
SoCs. Add YAML schema for its bindings.

Signed-off-by: Yangtao Li <tiny.windzz@gmail.com>
Signed-off-by: Vasily Khoruzhick <anarsoul@gmail.com>
Acked-by: Maxime Ripard <mripard@kernel.org>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/20191219172823.1652600-3-anarsoul@gmail.com