staging: dgap: adds dgap driver to staging

This patch adds the dgap driver to staging. This is
a TTY serial port driver for the EPCA PCI based
product line by Digi International <www.digi.com>.

Signed-off-by: Lidza Louina <lidza.louina@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>