iio: adc: ad7606: Move oversampling and scale options to chip info

The device dependent options which are going to be different for devices
which will be supported  in the future by this driver,
were moved in chip info for a more generic driver. This patch allows
supporting more devices by the driver. Also, it is an intermediate
step of adding support for ad7616 in software mode.

Signed-off-by: Beniamin Bia <beniamin.bia@analog.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>