Quadrature encoders, such as rotary encoders and linear encoders, are
devices which are capable of encoding the relative position and
direction of motion of a shaft. This patch introduces several IIO
constants for supporting quadrature encoder counter devices.

  IIO_COUNT: Current count (main data provided by the counter device)
  IIO_INDEX: Counter device index value
Signed-off-by: William Breathitt Gray <vilhelm.gray@gmail.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>