IIO: DDS: AD9834: Add support for AD9837 and AD9838 DDS devices

Add support for AD9837 and AD9838 DDS devices
Update copyright and license notice
Fix typo

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>