dt-bindings: iio: adc: ti,tsc2046: add vref-supply property

Add property for the voltage reference supply.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Acked-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Link: https://lore.kernel.org/r/20220904100203.3614502-1-o.rempel@pengutronix.de


Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>