iio: Rename iio/dds to iio/frequency

Generalize naming to allow other frequency synthesis techniques as well.
No functional changes.

Signed-off-by: Michael Hennerich <michael.hennerich@analog.com>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>