This patch adds calibration for the Meson SAR ADC.
Points 25% vref and 75% vref are used for calibration.
It uses a simple linear calibration function: SCALE * val + BIAS

Successfully tested on a Odroid C2.
Signed-off-by: NHeiner Kallweit <hkallweit1@gmail.com>
Tested-by: NMartin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: NMartin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>