iio: adc: ad4030: Add support for ADAQ4216 and ADAQ4224

iio: adc: ad4030: Add support for ADAQ4216 and ADAQ4224

ADAQ4216 and ADAQ4224 are similar to AD4030, but feature a PGA circuitry
that scales the analog input signal prior to it reaching the ADC. The PGA
is controlled through a pair of pins (A0 and A1) whose state define the
gain that is applied to the input signal.

Add support for ADAQ4216 and ADAQ4224. Provide a list of PGA options
through the IIO device channel scale available interface and enable control
of the PGA through the channel scale interface.

Signed-off-by: Marcelo Schmitt <marcelo.schmitt@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>