iio: Implement counter channel type and info constants

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>