iio: buffer: Add support for REPEAT mode buffers

Buffers in REPEAT mode are similar to CYCLIC buffers, but are only repeated as
long as no new buffer has been submitted. As soon as a new buffer is submitted
when a REPEAT mode buffer is active the active buffer will be stopped and the
new one will be started.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>