iio: adc: ad7923: support extended range

By default the driver was always setting the RANGE bit which means that
the analog input goes from 0 to VREF. However, we might want to have 0
to 2xVREF. This change adds a new Firmware property to allow for the
extended range while keeping the default behavior if nothing is provided.

Signed-off-by: Nuno Sá <nuno.sa@analog.com>
Link: https://lore.kernel.org/r/20220912081223.173584-3-nuno.sa@analog.com


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