staging: Add dgrp driver for Digi Realport devices

This is based on dgrp-1.9 available from
ftp://ftp1.digi.com/support/beta/linux/dgrp/dgrp-1.9.tgz

Signed-off-by: Bill Pemberton <wfp5p@virginia.edu>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>