Device Sink: Add a "interpolation" parameter

This parameter can be used when the minimum sample rate of the hardware
is already too high.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
FMCOMMS2 Sink: Add a "interpolation" parameter

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
IIO examples: Use decimation in IIO sink of FM transmitter

This is required to avoid dropping samples when running directly on a
ZedBoard, as 1.536 MSPS is too fast for it.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>