iio: adc: tsc2046: add vref support

If VREF pin is attached, we should use external VREF source instead of
the internal. Otherwise we will get wrong measurements on some of the channel
types.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20220904100203.3614502-2-o.rempel@pengutronix.de


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