iio: adc: ina2xx: Allow setting Shunt Voltage PGA gain and Bus Voltage range

Reducing shunt and bus voltage range improves the accuracy, so allow
altering the default settings.

Both settings are exposed as gain values. While for the shunt voltage
this is straightforward, the bus range settings of 32V (default) and 16V
are mapped to gain values of 1 resp. 2, to provide a uniform API to
userspace.

As the gain settings are incorporated into the raw values by the sensor
itself, adjusting of the scale attributes is not necessary.

Signed-off-by: Stefan Brüns <stefan.bruens@rwth-aachen.de>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>