IIO Blocks: Set IO type as 'short'

This greatly reduces the complexity of the code as well as the global
CPU usage.

Signed-off-by: Paul Cercueil <paul.cercueil@analog.com>
Device Source: Add a "decimation" parameter

This 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 Source: Add a "decimation" parameter

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