staging: iio: Move sensor drivers to sub menus

We have a lot of drivers now, so the iio sub menu becomes quite large.
This patch creates sub menus for the different sensors.

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