iio: Add helper function for initializing triggered buffers

Add a helper function for executing the common tasks which are usually involved
in setting up a simple software ringbuffer. It will allocate the buffer,
allocate the pollfunc and register the buffer.

Signed-off-by: Lars-Peter Clausen <lars@metafoo.de>
Acked-by: Jonathan Cameron <jic23@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>