iio: core: Introduce DISTANCE channel type

Some devices export an estimation of the distance the user has covered
since the last reset.

One of this devices is Freescale's MMA9553L
(http://www.freescale.com/files/sensors/doc/ref_manual/MMA9553LSWRM.pdf)
that computes the distance based on the stride length and step rate.

Introduce a new channel type DISTANCE to export these values.

Signed-off-by: Irina Tirdea <irina.tirdea@intel.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>