Some devices export the current speed value of the user.

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

Introduce a new channel type VELOCITY and a modifier for the magniture or
norm of the velocity vector, IIO_MOD_ROOT_SUM_SQUARED_X_Y_Z.
Signed-off-by: NIrina Tirdea <irina.tirdea@intel.com>
Signed-off-by: NJonathan Cameron <jic23@kernel.org>