iio: adc: ad7606: Move common channel definition to header

iio: adc: ad7606: Move common channel definition to header

The common channel definition which are going to be used by both core
file but also spi file, were moved in header file. Some devices have
different channel definitions when are used in software mode, feature
available only with spi, and those definitions will be added in spi file.

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