iio: adc: ad7606: move software functions into common file

iio: adc: ad7606: move software functions into common file

Since the register are always the same, whatever bus is used, moving the
software functions into the main file avoids the code to be duplicated
in both SPI and parallel version of the driver.

Signed-off-by: Guillaume Stols <gstols@baylibre.com>
Co-developed-by: Angelo Dureghello <adureghello@baylibre.com>
Signed-off-by: Angelo Dureghello <adureghello@baylibre.com>
Link: https://patch.msgid.link/20250210-wip-bl-ad7606_add_backend_sw_mode-v4-3-160df18b1da7@baylibre.com
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>