iio: adc: tsc2046: add .read_raw support

Add read_raw() support to make use of iio_hwmon and other iio clients.

Signed-off-by: Oleksij Rempel <o.rempel@pengutronix.de>
Link: https://lore.kernel.org/r/20220117082852.3370869-1-o.rempel@pengutronix.de


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