Ring buffer support within IIO

This document is intended as a general overview of the functionality
a ring buffer may supply and how it is specified within IIO.  For more
specific information on a given ring buffer implementation, see the
comments in the source code.  Note that the intention is to allow
some drivers to specify ring buffers choice at probe or runtime, but
for now the selection is hard coded within a given driver.

A given ring buffer implementation typically embedded a struct
iio_ring_buffer and it is a pointer to this that is provided to the
IIO core. Access to the embedding structure is typically done via
container_of functions.

struct iio_ring_buffer contains 4 function pointers
(preenable, postenable, predisable, postdisable).
These are used to perform implementation specific steps on either side
of the core changing it's current mode to indicate that the ring buffer
is enabled or disabled (along with enabling triggering etc as appropriate).

Also in struct iio_ring_buffer is a struct iio_ring_access_funcs.
The function pointers within here are used to allow the core to handle
as much ring buffer functionality as possible. Note almost all of these
are optional.

mark_in_use, unmark_in_use
  Basically indicate that not changes should be made to the ring
  buffer state that will effect the form of the data being captures
  (e.g. scan elements or length)

store_to
  If possible, push data to ring buffer.

read_last
  If possible get the most recent entry from the buffer (without removal).
  This provides polling like functionality whilst the ring buffering is in
  use without a separate read from the device.

rip_lots
  The primary ring buffer reading function. Note that it may well not return
  as much data as requested.  The deadoffset is used to indicate that some
  initial data in the data array is not guaranteed to be valid.

mark_param_changed
  Used to indicate that something has changed. Used in conjunction with
request_update
  If parameters have changed that require reinitialization or configuration of
  the ring buffer this will trigger it.

get_bpd, set_bpd
  Get/set the number of bytes for a given reading (single element, not sample set)
  The value of bps (bytes per set) is created from a combination of this and the
  enabled scan elements.

get_length / set_length
  Get/set the number of sample sets that may be held by the buffer.

is_enabled
  Query if ring buffer is in use
enable
  Start the ring buffer.