iio: core: Introduce IIO_EV_DIR_NONE

For some events (e.g.: step detector) a direction does not make sense.

Add IIO_EV_DIR_NONE to be used with such events and generate sysfs event
attributes that do not contain direction.

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