staging:iio: implement an iio_info structure to take some of the constant elements out of iio_dev.

This was suggested by Arnd Bergmann,  Other elements may well
move in here in future, but it definitely makes sense for these.

Signed-off-by: Jonathan Cameron <jic23@cam.ac.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>