dt-bindings: power: supply: bq256xx: Add ti,no-thermistor property

Add a new property ti,no-thermistor to indicate that no thermistor is
connected to the TS pin of the chip.

Signed-off-by: Hermes Zhang <chenhuiz@axis.com>
Reviewed-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>