Staging: IIO: DDS: AD9832 / AD9835 driver

This is a complete rewrite of the AD9832/35 driver.
Purpose was to move this driver to the recently
created API for such devices.

Changes since V1:
IIO: DDS: AD9832 / AD9835 driver: Apply review feedback

Save a few bytes, use union for data allocated for spi buffers.
Remove use of device IDs.
Fix comments.
Make master clock mclk always type unsigned long.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>