iio: adc: Add scaling support to exynos adc driver

Currently the driver only exposes the raw counts.  As we
have the regulator voltage and the maximum value (stored in
the data mask), we can trivially produce a scaling fraction
of voltage / max value.

This assumes that the regulator voltage is in fact the max
voltage, which appears to be the case for all mainline dts
and cross referenced with the public Exynos4412 and S5PV210
datasheets.

Signed-off-by: Jonathan Bakker <xc-racer2@live.ca>
Reviewed-by: Krzysztof Kozlowski <krzk@kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>