iio: dummy: Convert IIO dummy to configfs

We register a new device type named "dummy", this will create a
configfs entry under:
	* /config/iio/devices/dummy.

Creating dummy devices is now as simple as:

$ mkdir /config/iio/devices/dummy/my_dummy_device

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