dt-bindings: thermal: loongson,ls2k-thermal: Fix binding check issues

Add the missing 'thermal-sensor-cells' property which is required for
every thermal sensor as it's used when using phandles.
And add the thermal-sensor.yaml reference.

In fact, it was a careless mistake when submitting the driver that
caused it to not work properly. So the fix is necessary, although it
will result in the ABI break.

Fixes: 72684d99 ("thermal: dt-bindings: add loongson-2 thermal")
Cc: Yinbo Zhu <zhuyinbo@loongson.cn>
Signed-off-by: Binbin Zhou <zhoubinbin@loongson.cn>
Reviewed-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Link: https://lore.kernel.org/r/6d69362632271ab0af9a5fbfa3bc46a0894f1d54.1700817227.git.zhoubinbin@loongson.cn