In this patch restructures the existing ad5686 driver by adding a module
for SPI and a header file, while the baseline module deals with the
chip-logic.

This is a necessary step, as this driver should support in the future
similar devices which differ only in the type of interface used (I2C
instead of SPI).
Signed-off-by: NStefan Popa <stefan.popa@analog.com>
Signed-off-by: NJonathan Cameron <Jonathan.Cameron@huawei.com>