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>
diff --git a/drivers/iio/industrialio-core.c b/drivers/iio/industrialio-core.c
index 8d2c9ba..655755b 100644
--- a/drivers/iio/industrialio-core.c
+++ b/drivers/iio/industrialio-core.c
@@ -73,6 +73,7 @@
 	[IIO_ACTIVITY] = "activity",
 	[IIO_STEPS] = "steps",
 	[IIO_ENERGY] = "energy",
+	[IIO_DISTANCE] = "distance",
 };
 
 static const char * const iio_modifier_names[] = {