staging: iio: adis16400 add _type attributes for all scan elements

The bit depth of 14 for the supply adc on the datasheet is highly
suspicious.  Confirmation requested from Analog.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Manuel Stahl <manuel.stahl@iis.fraunhofer.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>