iio: adc: ti_am335x_adc: Add the scale information

Without the scale, the values returned to the user are just a picture of
the input voltage against the full scale range of the ADC. We need to
provide the actual conversion factor to get milli-Volts values.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Acked-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20211015081506.933180-45-miquel.raynal@bootlin.com