iio: adc: ad7606: Move spi dependent features to spi file

Because software mode and register access are only available in spi, they
were moved in spi file and are accessbile via bops structure.
The write_os/scale will be overwritten by sw_mode_config function.
This patch was made in order to support devices in software mode without
making the driver dependent to spi and increase the abstraction of the
core.

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