staging:iio: Add an iio channel type string to support proximity sensors.

Add "proximity" to the iio_chan_type_name_spec_shared string list to
support proximity sensors.
Now this list fully matches the declared iio_chan_type enums.

Signed-off-by: Bryan Freed <bfreed@chromium.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>