iio: adc: ad7606: Add support for software mode for ad7616

Support for ad7616 running in software was added. In order
to activate the software mode, HW_RNGSEL pins must be pulled low.
Oversampling and input ranges are now configured in corresponding
registers. Ad7616 has multiple scale options when it is configured
in software mode.
Also, in order to support multiple devices in software mode, the spi
calculation of registers address must be generic. Because
the length of address and bit which specifies the read/write operation is
different for every device, calculation of address was made generic.

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