Staging: IIO: Add generic ring buffer support to the IIO core

This provides a unified interface for hardware and software
ring buffers.

Changes since V2:
* Moved to a more consistent structure.  Now the ring buffer
  has an associated struct device which is a child of the
  relevant iio_dev.  This in turn has two children, one
  for the event interface and one for the access interface.
  These two interfaces are now managed via cdev structures.

* Numerous minor cleanups

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>