iio: Add new operating mode for non triggered sw buffers

There was a need for non triggered software buffer type.  It can be used when
triggered model does not fit and INDIO_BUFFER_HARDWARE causes confusion because
the data stream can be obtained not directly form hardware backend.

Suggested-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Karol Wrona <k.wrona@samsung.com>
Reviewed-by: Lars-Peter Clausen <lars@metafoo.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>