staging:iio: filter description - low pass 3db frequency.

Introduce the first data filtering related parameter.
For now we are ignoring the filter type and merely specifying its
approximate (I read them off tiny graphs) 3db point.

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